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REESE  LIBRARY 


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LIBRARY 
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PLATE   I. 


III 

g.  1  Fig.  2  Fig.  3 


Fig.  4 


A 

COURSE 


IN 


EXPERIMENTAL  PSYCHOLOGY 


BY 


EDMUND  C.  SANFORD,  PH.D. 

ASSISTANT  PROFESSOR  OF  PSYCHOLOGY, 
CLARK  UNIVERSITY. 


PART  I:  SENSATION  AND   PERCEPTION. 


BOSTON,  U.S.A.: 

D.   C.   HEATH  &   CO.,   PUBLISHERS. 
1897. 


53 

BIOLOGY 

LIBRARY 

G 


COPYRIGHT,  1894, 
BY  EDMUND  C.  SANFOBD. 


TYPOGRAPHY  BY  C.  J.  PETEBS  &  SON,  BOSTON. 


PBESSWOBK  BY  S.  J.  PABKHILL  &  Co.,  BOSTON. 


PBEFATORY   NOTE 

TO 
EDITION   OF   ADVANCED   SHEETS. 


THE  portion  of  the  course  which  follows  will  be  found  to 
treat  of  the  senses  only,  and  indeed  not  fully  of  them,  for 
it  still  lacks  a  chapter  upon  some  of  the  most  interesting 
experiments  in  vision.  The  author's  excuse  for  allowing 
the  publication,  even  in  this  modest  form,  of  so  incomplete 
a  work,  must  be  the  very  extraordinary  condition  of  experi- 
mental psychology  at  this  time.  Many  laboratories  have 
been  opened,  and  many  teachers  of  psychology  are  anxious 
to  give  their  students  the  benefit  of  demonstrations  and 
practice  work,  and  yet  there  is  absolutely  no  laboratory 
handbook  of  the  subject  to  be  had.  At  such  a  time  half  a 
loaf  may  be  better  than  110  bread  —  at  least,  so  a  number  of 
the  author's  professional  friends  have  seemed  to  believe; 
and,  since  the  completion  of  the  whole  must  be  still  further 
delayed,  he  offers  this  half  loaf. 

The  course  as  planned  consists  of  two  parts :  PART  I  on 
sensation  and  perception ;  and  PART  II  on  more  complex 
mental  phenomena. 

PART  I  needs  three  chapters  more  to  complete  it :  Chap- 
ter VII,  on  the  Visual  Perception  of  Extent,  Distance, 
Direction,  and  Motion;  Chapter  VIII,  On  the  Psycho- 
physic  Methods  and  Weber's  Law;  and  Chapter  IX,  On 
Apparatus  for  the  Study  of  the  Senses.  PART  II  will  con- 
tain chapters  on  the  following  topics  :  Eeflex  and  Voluntary 

iii 


iy  PREFATORY  NOTE. 

Movement,  The  Time  Relations  of  Mental  Phenomena, 
Association,  Memory,  Attention,  and  Emotion,  so  far  as 
these  subjects  can  be  approached  with  experiments  of  mod- 
erate difficulty,  together  with  a  chapter  on  the  apparatus 
necessary  for  such  experiments. 

E.  C.  S. 
WORCESTER,  July^  1894. 


LABORATORY  COURSE  IN  PSYCHOLOGY. 


CHAPTER    I. 
The  Dermal  Senses. 

THE  sense  organs  of  the  skin  give  us  besides  pain,  tick- 
ling, shudder,  and  the  like,  the  more  special  sensations  of 
contact,  heat,  cold,  and  pressure.  All  these  may  be  received 
passively  when  our  members  are  at  rest,  or  actively  when 
our  members  are  in  motion,  in  which  case  special  sensations 
of  motion  are  blended  with  those  just  mentioned.  We  also 
assign  to  each  sensation  a  more  or  less  exact  location.  To 
examine  some  of  these  skin  sensations  is  the  purpose  of  this 
chapter.1 

SENSATIONS  OF  CONTACT. 

1.  The  Location  of  Touches.  Touch  yourself  in  several 
places  with  the  same  object,  and  analyze  out,  as  far  as  you 
can,  the  particular  quality  of  the  sensation  by  which  you 
recognize  the  place  touched.  This  quality  of  a  sensation  is 
known  as  its  "  Local  Sign." 

Lotze,2  A,  328  ff.,  405  ff.  ;  J5,  39  ff.     Stumpf. 

1  As  a  general  term  for  perceptions  of  touch  in  the  widest  sense,  Max  Dessoir 
(p.  242)  suggests  Haptics  as  an  analogue  of  Optics  and  Acoustics.    This  he  further 
divides  into  Contact-sense  (including  a,  pure  contact,  and  6,  pressure)  and  Psela- 
phesia,  from  i//T)Aa<J>T7<n?,  touching,  handling  (including  a,  active  touch,  and  6, 
"muscle  sense"). 

2  For  full  titles  of  books  and  articles  referred  to,  see  the  bibliography  at  the 
end  of  the  chapter.    When  several  articles  from  one  author  are  given,  they  have 
been  lettered  A,  Bt  C,  etc.,  and  the  references  marked  accordingly. 

i 


2  LABORATORY  COURSE  IN  PSYCHOLOGY. 

2.  Location  of  Touches.     Cause  the  subject  to  close  his 
eyes ;  touch  him  on  the  fore-arm  with  a  pencil  point ;  and 
require  him  to  touch  the  same  point  with  another  pencil 
immediately  afterward.      Estimate  the  error  in  millimetres 
and  average  the  results  for  a  number  of  trials,  noting  the 
direction  of  error,  if  it  is  constant.     The  subject  must  be 
allowed  to  correct  his  placing  of  the  pencil  if  not  satisfied 
with  it  on  first  contact. 

3.  Aristotle's  Experiment.     Cross  the  middle  finger  over 
I  x  the  first  in  such  a  way  as  to  bring  the 

\  tip  of  the  middle  finger  on  the  thumb 

/  \         side  of  the  first  finger.     Insert  between 

the  two  a  pea  or  other  small  object.  A 
more  or  less  distinct  sensation  of  two 
objects  will  result,  especially  when  the 
fingers  are  moved.  Some  experimenters 
may  find  the  illusion  more  marked  when 
the  pea  is  rolled  about  on  the  surface  of 
the  table  with  the  crossed  fingers,  or 
when  the  third  and  little  fingers  are  used 
instead  of  the  first  and  middle  fingers. 
Aristotle,  Hoppe,  James,  II.,  86-87. 

4.  Eccentric  Projection  of  Touches.     Close  the  eyes,  and 
tap  with  the  tip  of  a  cane  on  the  floor,  or,  better  still,  on 
the  walls  and  floor  near  a  corner  of  the  room.     Notice  that 
the  origin  of  the  sensations  seems  to  be  the  tip  of  the  cane 
and  not  the  fingers  or  the  arm.     Attention  to  these  parts, 
however,  will  show  the  true  place  of  origin.     If  the  cane  is 
held  rigidly  at  the  lower  end,  there  is  little  or  no  tendency 
to  shift  the  sensations  from  the  fingers  and  arm,  unless  the 
cane  is  limber.     The  eccentric  projection  of  touches  is  only 
a  special  case  of  their  location,  and  follows  the  same  general 
laws.     See  also  Ex.  41. 

Weber,  483  f.;  James,  II.,  31-43,  195-197;  Dessoir,  219-232. 


TEE  DERMAL  SENSES.  3 

5.  Judgments  of  Motion  on  the  Skin.     a.   Let  the  sub- 
ject close  his  eyes.    Rest  a  pencil  point  or  the  head  of  a  pin 
gently  on»  his  fore-arm  and  move  it  slowly  and  evenly  up  or 
down  the  arm.     Bequire  him  to  indicate  his  earliest  judg- 
ment of  the  direction.     If  the  experiment  is  carefully  made, 
the  fact  of  motion  will  be  perceived  before  its  direction. 

b.  Try  a  number  of  times,  estimating  the  distances  trav- 
ersed in  millimetres  and  averaging  for  the  two  directions 
separately.     It  will  probably  be  found  that  the  downward 
distances  have  been  greater  than  the  upward. 

c.  Starting  from  a  fixed  point  on  the  fore-arm,  move  the 
pencil  in  irregular  order  up,  down,  right,  or  left,  and  require 
the  subject  to  announce  the  direction  of  motion  as  before. 
Compare  the  results  found  with  those  found  in  Ex.  7. 

Hall  and  Donaldson. 

6.  Feelings  of  Double  Contact,     a.   If  two  parts  of  the 
body  of  like  temperature  are  brought  in  contact,  the  two 
sensations  do  not  blend,  but  the  part  that  moves  feels  the 
one  that  does  not ;   i.  e.,  the  sensations  received  by  the  mov- 
ing part  generally  get  more  attention  and  are  externalized. 
Try  with  the  tips  of  the  thumbs  or  fingers  in  contact.    This 
general  rule,  however,  has  exceptions.     Feel  of  the  palm  of 
the  right  hand  first  with  the  ball  of  the  left  thumb  (which 
gives   results    in    accord   with   the    rule),   then  with   the 
knuckle  of  the  same  thumb  sharply  bent.     Light  tapping 
of  the  forehead  with  the  finger  we  feel  in  the  forehead  more 
markedly  than  in  the  finger,  though  usually  with  the  hand 
on  the  forehead  we  feel  the  forehead. 

b.  If  the  parts  are  not  of  like  temperature  that  which 
varies  most  from  the  normal  bodily  temperature  will  be  felt 
by  the  other.  Warm  the  right  hand  by  holding  it  closed 
for  a  minute  or  two  and  then  apply  it  to  the  forehead.  The 
higher  temperature  will  be  perceived  by  the  forehead,  while 


4  LABORATORY  COURSE  IN  PSYCHOLOGY. 

at  the  same  time  the  hand  as  the  more  expert  touch  organ 
will  perceive  the  form  of  the  forehead.  Cool  the  right  hand 
by  holding  it  a  few  minutes  in  cold  water,  dry  it  and  apply 
it  to  the  back  of  the  left  hand.  The  right  hand  may  seem 
to  be  feeling  of  a  cold  left  hand.  In  this  case  of  course 
both  the  temperature  and  form  feelings  are  credited  to  the 
right  hand.  If  the  temperature  is  not  very  different  the 
direction  of  attention  may  dictate  which  shall  be  felt  by 
the  other. 

Weber,  556-559  ;  Dessoir,  229. 

7.  Weber's  Sensory  Circles,  a.  Find  the  least  distance 
apart  at  which  the  points  of  the  aesthesiometric  compasses l 
can  be  recognized  as  two  when  applied  to  the  skin  of  the 
fore-arm.  Try  also  the  upper  arm,  the  back  of  the  hand, 
the  forehead,  the  finger-tip,  and  the  tip  of  the  tongue.  Be 
very  careful  to  put  both  points  on  the  skin  at  the  same  time 
and  to  bear  on  equally  with  both.  Cf.  Weber's  measure- 
ments as  given  in  the  text-books ;  also  Goldscheider's 
(quoted  by  Ladd,  p.  411). 

b.  Compare  the  distance  between  the  points  just  recog- 
nizable as  two  when  applied  lengthwise  of  the  arm  with 
that  found  when  they  are  applied  crosswise.     Compare  the 
results  found  in  a  and  b  with  those  found  in  Ex.  5,  but 
remember  that  this  compass  experiment  requires  the  dis- 
crimination of  the  points. 

c.  Give  the  points  a  slightly  less  separation  than  that 
found   for  the   fore-arm   crosswise,  and   beginning   at   the 
elbow  draw  the  points  downward  side  by  side  along  the 
arm.     They  will  at  first  appear  as  one,  later  as  two,  after 
which  they  will  appear  to  separate  as  they  descend.     Some- 
thing similar  will  be'  found  on  drawing  the  points  from  side 

1  For  the  apparatus  needed  in  this  and  later  experiments,  see  the  list  and 
descriptions  in  the  chapter  on  apparatus  below. 


THE  DERMAL  SENSES.  5 

to  side  across  the  face  so  that  one  shall  go  above,  the  other 
below  the  mouth. 

d.  Make  the  skin  anaesthetic  with  an  ether  spray  and  test 
the  discriminative  sensibility  as  before. 

Weber,  524-530,  536-541;  Goldscheider,  B,  70  ff.,  84  ff. 

8.  Filled    Space    is   relatively  under-estimated  on    the 
skin.     Set   up  in  a   small  wooden  rod  a  row  of   five  pins 
separated  by  intervals  of  half  an  inch,  and  in  another  two 
pins   two   inches   apart.     Apply  to  the  arm  like  the  com- 
passes above.     The  space  occupied  by  the  five   pins  will 
seem  less   than  that  between  the   two.     A   still   simpler 
way   given  by   James    is    as   follows:  Cut   one   end  of   a 
visiting  card  into  a  series  of  notches,  and  the  other  into 
one  long  notch  so  as  to  leave  two  points  as  far  apart  as 
the  outer  points   at  the   other   end,   but  separated  by  an 
empty  interval.     Apply  to  the  skin  as  before.     This  illu- 
sion, though  very   clear  for  some  experimenters,  does  not 
seem   equally  so  for    all,    and  some   have   difficulty  with 
it. 

James,  II.,  141,  footnote. 

9.  Active  Touch  is  far  more  discriminating  than  mere 
contact.     Compare    the   sensations   received   from    simply 
resting  the  tip  of  the  finger  on  a  rough  covered  book  with 
those  received  when  the  finger  is  moved  and  the   surface 
«  felt  of ." 

10.  The  Time  Discriminations  of  the  sense  of  contact  are 
very  delicate.     Strike  a  turiing-f ork ;  touch  it  lightly,  and 
after  about  a  second  remove  the  finger  so  as  not  to  stop  the 
fork.     The  taps  of  the  fork  on  the  skin  do  not  blend  into  a 
smooth  sensation  even  when  the  vibrations  are  several  hun- 
dred a  second.     One  may  assure  himself  that  the  touching 
does  not  much  alter  the  rate  of  the  fork  by  using  another 
that  beats  with  the  first.     If  the  touching  is  carefully  done, 


6  LABORATORY   COURSE  IN  PSYCHOLOGY. 

the  rate  of  the  beats  will  not  be  noticeably  altered.  (On 
beating  forks  see  Chap.  IV.)  The  roughness  may  also  be 
felt  but  not  so  strongly,  by  setting  the  stem  of  the  fork 
upon  the  skin.  The  roughness  of  the  pulses  of  air  from 
large  tuning-forks  can  also  be  felt  when  the  hand  is  brought 
near,  but  not  into  actual  contact  with  them. 
Wittich,  335  ff.;  Schwaner;  Sergi. 

11.  After-images  of  Touch.     Touch  the  skin  of  the  wrist 
lightly  with  the  point  of  a  needle,  and  notice  that  beside 
the  original  sensation,  there  is,  after  a  more  or  less  free 
interval,  a  second  pulse   of    sensation.      The   interval  is 
brief,  a  second  or  under,  and  the  sensation  appears  to  come 
from  within.     In  quality  it  is  like  the  first,  but  without  the 
pressure  component.     The  prick  of  the  needle  point  is  not 
essential ;  the  second  sensation  can  be  observed  when  the 
head  of  a  pin  is  applied.    Too  hard  touches  must  be  avoided 
in  testing  for  these  images,  as  they  give  rise  to  a  continuous 
after-image  that  fills  the  interval.     The  second  image  is 
apparently  due  to  a  double  conduction  in  the  spinal  cord, 
and  is  therefore  different  from  the  after-images  of  the  other 
senses.     A  portion  of  the  original  excitation  is  conveyed  in 
the  posterior  columns  of  the  cord  to  the  cortex.     Another 
portion  goes  by  a  slower  path  through  the  central  gray 
matter  of  the  cord.     Of.  Ex.  32. 

Goldscheider,  H,  168  f. 

12.  An  Interesting  Illusion  of  Length,  based  on  the  time 
during  which  a  touch  sensation  continues,  may  be  observed 
as  follows :   Require  the  subject  to  close  his  eyes.     Take  a 
piece  of  coarse  thread  a  couple  of  feet  long  and  make  a  knot 
in  the  middle  of  it.     Place  the  knot  between  the  thumb  and 
forefinger  of  the  subject,  asking  him  to  press   it   gently. 
Then  draw  the  thread  slowly  through  between  his  thumb 
and  finger  and  ask  him  to  estimate  its  length.     Eepeat  the 


THE  DERMAL  SENSES.      .  7 

process,  this  time  drawing  it  rapidly.  The  drawing  must 
not  be  too  slow  in  the  first  case  nor  too  fast  in  the  second, 
or  the  nature  of  the  illusion  may  be  suggested  to  the  sub- 
ject and  more  or  less  completely  corrected. 

Loeb,  121-122. 

For  Minimal  Contact  in  relation  to  Pressure,  see  Ex.  22  ; 
in  relation  to  Tickle,  see  Ex.  31. 

SENSATIONS  OF  TEMPERATURE. 

13.  Hot  and  Cold  Spots,  a.  Move  one  of  the  pointed 
brass  rods,  or  even  a  cool  lead-pencil,  slowly  and  lightly 
over  the  skin  of  the  back  of  the  hand.  At  certain  points 
distinct  sensations  of  cold  will  flash  out,  while  at  others  no 
temperature  sensation  will  be  perceived,  or,  at  most,  only 
faint  and  diffuse  ones.  Heat  one  of  the  rods  slightly  in 
the  gas  flame  and  repeat  the  experiment.  More  care  will 
be  required  in  locating  the  hot  spots  than  the  cold  spots, 
for  their  sensations  seem  less  distinct. 

b.  On  some  convenient  portion  of  the  skin  mark  off  the 
corners  of  a  square  2  cm.  on  the  side.  Go  over  this  square 
carefully  both  lengthwise  and  crosswise  for  both  heat  and 
cold,  drawing  the  point  along  lines  1mm.  apart,  and  note  on 
a  corresponding  square  of  millimetre  paper  the  hot  and  cold 
spots  found,  hot  spots  with  red  ink,  cold  with  black.  This 
time  the  points  should  be  heated  or  cooled  considerably  by 
placing  them  in  vessels  of  hot  or  cold  water,  and  should  be 
kept  at  an  approximately  constant  temperature  by  frequent 
change,  one  being  left  in  the  water  while  the  other  is  in  use. 
Break  the  experiment  into  a  number  of  sittings  so  as  to 
avoid  fatiguing  the  spots,  for  they  are  very  easily  fatigued. 
A  map  made  in  this  way  cannot  hope  to  represent  all  the 
spots,  but  it  will  suffice  to  show  the  permanence  of  some  of 
them  and  possibly  to  show  a  little  their  general  arrange- 
ment. When  the  map  has  been  made,  select  a  responsive 


8  LABORATORY  COURSE  IN  PSYCHOLOGY. 

and  isolated  cold  spot,  and  try  it  with  a  warm  point.  Try 
a  similar  hot  spot  with  a  cold  point. 

c.  Notice  the  very  distinct  persistence  of  the  sensations 
after  the  point  has  been  removed,  that  is,  the  temperature 
after-images. 

An  interesting  question  suggested  by  this  punctual  loca- 
tion of  temperature  sensations  is  this,  namely :  How  does  it 
come  about  that  we  ordinarily  conceive  such  sensations  as 
continuous  over  considerable  areas. 

Blix;  Goldscheider,  A,  B,  E ;  Donaldson. 

14.  Mechanical  and  Chemical  Stimulation  of  the  Temper- 
ature Spots.1  The  temperature  spots  respond  with  their 
characteristic  sensations  to  mechanical  and  chemical  stimu- 
lation (and  some  observers  find  also,  to  electrical  stimula- 
tion), and  do  not  give  pain  when  punctured. 

a.  Choose  a  very  certainly  located  cold  spot  and  tap  it 
gently  with  a  fine  wooden  point  (not  too  soon  after  locating 
it,  if  it  has  been  fatigued  in  locating) ;  or  better,  have  an 
assistant  tap  it.     Thrust  a  needle  into  a  well-located  cold 
point.     Try  both  for  comparison  on  an  adjacent  portion  of 
the  skin. 

b.  Choose  a  convenient  area,  say,  on  the  back  of  the  hand 
or  the  temple,  and  rub  the  skin  lightly  with  a  menthol 
pencil.     After  a  little  the  sensation  of  cold  will  appear. 
Groldscheider's  tests  with  a  thermometer  applied  to  the  skin 
show  that  the  sensation  is  not  due  to  an  actual  cooling  of  it. 
The  menthol  makes  the  nerves  of  cold  at  first  hyperses- 
thetic  (so  that  they  respond  with  their  specific  sensation  to 

1  Such  experiments  as  these  illustrate  the  Law  of  the  Specific  Energy  of 
Nerves,  which  may  be  stated  somewhat  as  follows:  Every  stimulus  that  can 
excite  a  sensory  nerve  at  all,  causes  such  sensations  as  follow  the  stimulation  of 
that  nerve  in  its  customary  way  and  only  such.  As  regards  the  interpretation  to 
be  put  on  the  phenomena  thus  generalized  there  is  dispute.  Goldscheider  /; 
Wundt,  3te  Aufl.  I.  332  ff.,  4te  I.  Aufl.  323;  Helmholtz,  Sensations  of  Tone,  148  ; 
Optik,  2te  Aufl.  233,  Ite  Aufl.  193;  Ladd,  307,  353. 


THE  DERMAL  SENSES.  9 

mere  contact,  and  give  an  intenser  sensation  when  a  cold 
body  is  applied  than  do  adjacent  normal  portions  of  the 
skin)  ;  afterward,  however,  all  the  cutaneous  nerves  become 
more  or  less  anaesthetic. 

c.  Chemical  stimulation  of  the  heat  nerves  can  be  tested 
with  C02.  Provide  two  like  vessels ;  place  them  side  by 
side  and  fill  one  with  C02.  Plunge  the  hand  into  the  vessel 
containing  the  gas,  and  for  comparison  into  the  one  contain- 
ing air.  For  the  additional  experiments  necessary  to  prove 
this  to  be  a  real  chemical  stimulation,  see  the  literature. 

Blix,  Goldscheider  A,  J5,  D,  F,  and  Donaldson;  on  c,  R.  Du 
Bois-Reymond. 

15.  The  Temperature  of  the  Skin  at  any  moment  is  a 
balance  between  its  gain  and  loss  of  heat.     Anything  that 
disturbs  that  balance,  causing  increased  gain  or  loss,  pro- 
duces  temperature   sensations.     It  is   common  experience 
that  a  piece  of  cloth,  a  bit  of  wood,  a  piece  of  metal,  all  of 
the  same  temperature  as  the  air  that  seems  indifferent  to 
the  hand,  cause  different  degrees  of  the  sensation  of  cold 
when  touched,  because  they  increase  the  loss  of  heat  by  con- 
duction in  different  degrees.     If  a  paper  bag  be  placed  over 
the  hand  held  upward,  a  sensation  of  warmth  is  soon  felt, 
because  of  the  decreased  loss  of  heat. 

16.  The  Shifting  of  the  "  Physiological  Zero."     a.  Pro- 
vide three  vessels  of  water,  one  at  30°  C.,  the  second  at  40°, 
the  third  at  20°.     Put  a  finger  of  one  hand  into  the  warmer 
water,  a  finger  of  the  other  into  the  cooler.     At  first  the 
usual  temperature  sensations  will  be  felt,  but  after  a  little 
they  disappear  more   or  less   completely,  because   of  the 
fatigue   of    the   corresponding  temperature   organs.     Now 
transfer  both  fingers  to  the  water  of  normal  temperature. 
It  will  seem  cool  to  the  finger  from  warmer* water  and  warm 
to  the  one  from  cooler.     This  experiment  has  been  sometimes 
regarded  as  one  of  successive  contrast. 


10  LABORATORY  COURSE  IN  PSYCHOLOGY. 

b.  Hold  the  hand  for  one  minute  in  water  at  12°  C.,  then 
transfer  it  to  water  at  18°.  The  latter  will  at  first  feel 
warm,  but  after  a  time  cold  again.  The  water  at  18°  first 
causes  a  decrease  in  the  loss  of  heat  or  a  slight  gain,  but 
later  a  continued  loss. 

Weber;  Hering;  Goldscheider,  B,  32  ff. 

17.  Effect  of  Extent  of  Surface  Stimulated.     The  inten- 
sity of    the    sensation   increases  as  the   stimulated   area 
increases.     Dip  the  right  forefinger  (or  hand)  into  hot  or 
cold  water,  observe  the  sensation,  and  immediately  insert 
the  other  forefinger  to  an  equal  depth.     Vary  the  experiment 
by  inserting  the  left  finger  first,  and  by  inserting  both  at 
once  and  then  withdrawing  one.     The  original  experiment 
of  Weber,  who  inserted  first  a  finger,  and  then  the  whole  of 
the  other  hand,  gives  striking  results,  but  has  the  fault, 
as  Goldscheider  rightly  observes,  of  adding  a  more  sensitive 
as  well  as  a  larger  area.     This   experiment  must  not  be 
inconsiderately  contrasted  with  Ex.  23. 

Weber,  553;  Goldscheider,  G,  475-476. 

18.  Temperature  Fatigue,     a.   Extreme  temperatures  fa- 
tigue the  sensory  apparatus  of  both  heat  and  cold.     Hold  a 
finger  in  water  at  45°  C.,  the  corresponding  finger  of  the 
other  hand  in  water  which  feels  neither  cold  nor  hot  (about 
32°).     After  30  seconds  dip  them  alternately  into  water  at 
10°.     The  finger  from  the  water  at  32°  will  feel  the  cold 
more  strongly.     Hold  a  finger  in  water  at  10°,  the  corre- 
sponding finger  of  the  other  hand  in  water  at  32°.     After  30 
seconds  dip  them  alternately  in  water  at  45°.     The  finger 
from  the  water  at  32°  will  feel  the  heat  more  strongly. 

b.  The  fatigue  of  the  temperature  apparatus  may  produce 
an  apparent  contradiction  of  Ex.  17.  Phinge  one  hand 
entirely  under  cold  water  and  keep  it  there  for  a  moment. 
Then  dip  the  finger  of  the  other  hand  or  the  whole  hand 


THE  DERMAL  SENSES.  11 

several  times  in  the  same  water,  withdrawing  it  immediately 
each  time.  The  water  seems  colder  to  the  finger  or  hand 
which  is  only  dipped. 

Weber,  570;  Goldscheider,  B,  34  ff. 

19.  Temperature  After-images,     a.  Hold  a  cold  piece  of 
metal  on  the  forehead  or  on  the  palm  of  the  hand  for  half  a 
minute.     On  removing  it  the  sensation  of  cold  continues, 
though  the  actual  temperature  of  the  skin  is  rising.     Some- 
times fluctuations  are  observed  in  the  persisting  sensation. 
After  contact  with  a  hot  body  the  sensation  of  heat  con- 
tinues in  the  same  way,  though  the  temperature  of  the  skin 
falls.     Goldscheider  explains  this  result  for  cold  in  part  by 
the  persistence  of  the  cold  sensation  in  the  manner  of  an 
after-image,  and  in  part  by  the  lessened  sensibility  of  the 
nerves  of  heat;   a  similar  explanation   mutatis  mutandis 
holds  also  for  heat. 

b.  Intermittent  after-images,  or  those  that  recur  after  an 
interval  more  or  less  free  of  sensation,  have  been  observed 
especially  with  repeated  stimulation.  Heat  a  key  till  it  is 
just  a  little  short  of  painfully  hot,  touch  some  part  of  the 
skin,  e.g.,  the  wrist,  three  or  four  times  at  intervals  of  about 
half  a  second.  The  after-image  of  the  heat  will  appear 
several  seconds  later.  Try  the  same  for  cold,  but  use  a  key 
that  is  at  the  temperature  of  the  air. 

Cf.  Ex.  13  c.j  also  the  after-images  of  hearing  and  vision, 
Chapters  IV.  and  V.,  and  notice  that  all  the  temperature 
after-images  are  positive  ;  i.e.,  like  the  original  sensation. 

Goldscheider,  B,  11,  34  ff.,  38  ;    on  6,  Dessoir,  300. 

20.  Fineness   of   Temperature   Discrimination,     a.  Find 
what   is   the   least   perceptible    difference   in  temperature 
between  two  vessels  of  water  at  about  30°  C.,  at  about  0°, 
and  about  55°.     The  finest  discrimination  will  probably  be 
found  with  the  first  mentioned,  if  the  discrimination  does 


12           LABORATORY  COURSE  IN  PSYCHOLOGY. 

not  prove  too  fine  at  all  these  points  to  be  measured  with 
the  thermometers  at  hand.  Use  the  same  hand  for  these 
tests,  always  dipping  it  to  the  same  depth.  It  is  better  to 
dip  the  hand  repeatedly  than  to  keep  it  in  the  water. 

b.  The  different  surfaces  of  the  body  vary  much  in  their 
sensitiveness   to   temperature.      The   mucous   surfaces   are 
quite   obtuse.     When   drinking  a  comfortably  hot   cup   of 
coffee,  dip  the  upper  lip  into  it  so  that  the  coffee  touches 
the  skin  above  the  red  part  of  the  lip,  or  dip  the  finger  into 
it;  it  will  seem  burning  hot.     Plunge  the  hand  into  water 
at   5-10°  C.     The  sensation   of   cold  will  be  strongest  at 
first  on  the  back  of  the  hand  where  the  skin  is  thin,  but  a 
little  later  will  come  out  more  strongly  in  the  palm,  where 
it  will  continue  to  be  stronger  and  may  finally  approach 
pain. 

c.  The  middle  line  of  the  body  is  less  sensitive  to  tem- 
perature than  portions   at   either   side  of  it.     Touch  the 
middle  of  the  forehead,  or  the  tip  of  the  nose,  with  a  piece 
of  warm  or  cold  metal  and  then  touch  several  places  to  the 
right  and  left  of  that  point. 

Fechner;  Weber,  552  ff.;  Goldscheider,  B,  49  ff. 

SENSATIONS  OF  PRESSURE. 

21.  Pressure  Points.  Make  an  obtuse  but  extremely  fine 
cork  point  (pyramidal  in  shape ;  for  example,  the  pyramid  a 
quarter  of  an  inch  square  on  the  base  and  of  equal  height), 
set  it  upon  the  point  of  a  pen  or  other  convenient  holder,  or 
use  a  match  whittled  down  to  a  fine  point,  or  even  a  needle. 
Choose  an  area  on  the  fore-arm  and  test  for  its  pressure 
spots  somewhat  as  for  the  hot  and  cold  spots,  but  this  time 
set  the  cork  point  as  lightly  as  possible  on  point  after  point 
of  the  skin  instead  of  drawing  it  along.  Two  kinds  of  sen- 
sation will  be  felt ;  at  some  points  a  clear  feeling  of  contact 
with  a  sharp  point  will  be  felt,  at  others  no  feeling  at  all,  or 


THE  DERMAL  SENSES.  13 

a  dull  and  vacuous  one.  The  first  are  the  pressure  points. 
Goldscheider  describes  their  sensations  on  light  contact  as 
"  delicate/*  "  lively,"  "  somewhat  tickling  ...  as  from  mov- 
ing a  hair  ; "  on  stronger  pressure,  "  as  if  there  were  a  resist- 
ance at  that  point  in  the  skin,  which  worked  against  the 
pressure  stimulus ; "  "  as  if  a  small  hard  kernel  lay  there 
and  was  pressed  down  into  the  skin." 

The  first  are  said  to  be  more  sensitive  to  small  changes 
of  pressure,  and  though  with  sufficient  increase  both  give 
pain,  their  sensations  retain  their  characteristics.  They  are 
closer  together  than  the  temperature  spots,  and  harder  to 
locate.  The  fact  that  our  most  frequent  sensations  of  pres- 
sure are  from  surfaces  and  not  from  points  is  perhaps  the 
reason  it  is  difficult  at  first  to  recognize  a  pressure  quality 
in  these  sensations. 

Goldscheider,  B,  76  ff. 

22.  Minimal  Pressure  or  Simple  Contact.     Mnd  weights 
that  are  just  perceivable  on  the  volar  side  of  the  fore-arm 
and  on  the  tips  of  the  fingers.     Try  also,  if  convenient,  the 
temples,  forehead,  and  eyelids.     In  applying  the  weights, 
see  that  they  are  brought  down  slowly  upon  the  surface  of 
the  skin,  that  they  touch  equally  at  all  points,  and  that 
their  presence  is  not  betrayed  by  motion  of  the  weight  after 
it  touches  the  skin.     This  can  be  done  by  using  a  penholder 
or  small  rod,  with  its  tip  put  through  the  ring  of  the  weight, 
for  laying  it  on.     Compare  the  relative  sensibility  found  by 
this  method  with  that  found  with  Weber's  compasses  for 
the   same  parts  (Ex.  7)  and  note   that  the  latter  requires 
discrimination,  not  mere  perception.     See  also  Exs.  29  and 
31. 

Aubert  and  Kammler ;  Bloch. 

23.  Eelation  of  Apparent  Weight  to  Area  of   Surface 
Stimulated.     Test  with  the  equal  weights  of  unequal  size 


14  LABORATORY  COURSE  IN  PSYCHOLOGY. 

upon  the   hand,  properly  supported  to   exclude  "  muscle 
sense."     The  smaller  will  seem  decidedly  heavier. 

24.  Discriminative  Sensibility  for  Pressures.  Use  the 
pressure  balance  if  one  is  at  hand;  if  not,  have  the  subject 
close  his  eyes  and  lay  his  hand,  palm  upward,  on  such  a 
support  as  will  bring  his  arm  into  a  comfortable  position 
and  make  his  palm  level ;  for  example,  on  a  folded  towel 
placed  on  a  low  table  or  the  seat  of  a  chair.  (The  matter 
of  an  easy  position  for  the  subject  is  of  cardinal  importance 
in  all  psychological  experiments.)  The  method  of  experi- 
menting here  to  be  used  is  that  of  the  "  Just  Observable 
Difference  "  or  "  Minimal  Change  ; "  it  may  be  applied  as 
follows :  Lay  in  the  subject's  palm  a  piece  of  thick  and 
soft  blotting-paper  just  large  enough  to  prevent  the  weight 
from  touching  the  skin.  Place  the  standard  weight  of  100 
grams  upon  the  paper  and  allow  it  to  remain  a  sufficient 
time  for  the  subject  to  get  a  clear  perception  of  its  weight. 
Then  remove  it  and  immediately  put  in  its  place  a  weight  of 
110  grams,  allowing  that  to  remain  as  long  as  the  first.  If 
the  subject  can  recognize  this  difference  easily  and  surely,  try 
him  with  109, 108,  and  so  on,  alternating  the  standard  weight 
and  a  weight  to  be  compared  with  it  till  a  weight  is  found 
that  is  just  recognizably  different  from  the  standard.  If 
110  grams  is  not  recognizably  different,  take  111,  112  in- 
stead of  109,  108.  Occasionally  follow  the  standard  with 
another  100  gram  weight  to  guard  against  illusion  on  the 
part  of  the  subject.  After  having  determined  the  just 
observably  greater  weight,  find  the  one  that  is  just  observ- 
ably lighter  in  the  same  way.  Make  a  good  number  of 
determinations  of  these  just  observably  heavier  and  lighter 
weights,  sometimes  going  toward  the  standard  and  some- 
times away  from  it.  Take  the  differences  between  them  and 
the  standard  weight  and  average  the  results.  The  ratio  of 
this  average  to  the  standard  will  be  a  measure  of  the  dis- 


THE  DERMAL  SENSES.  15 

criminative  sensibility  required.  If,  for  example,  the  ratio 
for  one  subject  is  7 : 100  and  for  another  14: 100,  the  first 
has  a  sensibility  to  pressure  differences  twice  as  acute  as 
the  second.  In  half  of  the  tests,  both  above  and  below,  the 
standard  weight  must  be  placed  upon  the  hand  first,  and  in 
half  the  weight  to  be  compared  with  it.  It  is  well  also  to 
distribute  the  determinations  of  the  differences  above  and 
below  so  that  they  shall  be  about  equally  affected  by 
practice  and  fatigue.  The  aim  should  always  be  to  keep  all 
the  conditions  of  the  experiment  as  constant  as  possible 
and  especially  to  have  them  the  same  for  the  weights  to  be 
compared.  Be  careful  in  putting  on  the  weights  that  the 
subject  does  not  recognize  a  difference  in  the  force  with 
which  they  strike;  also  that  suggestions  by  difference  of 
temperature  or  by  sounds  made  in  selecting  the  weights 
are  avoided. 

It  is  easy  to  see  that  this  method  has  some  disadvantages. 
First,  it  leaves  to  the  feeling  of  the  subject  what  the  just 
observable  difference  is,  and  this  feeling  is  liable  to  change 
from  subject  to  subject  and  in  the  same  subject  at  different 
times.  In  using  this  method  the  subject  must  know  the 
direction  of  the  change  that  he  is  to  recognize,  and  so  is 
somewhat  exposed  to  the  influence  of  expectant  attention. 
And  finally,  when  weights  are  found  that  are  just  observ- 
ably different,  it  is  possible  that  they  are  a  little  larger  than 
the  subject  could  just  recognize ;  that  is,  that  he  has  allowed 
himself  a  small  margin  for  security.  These  difficulties  may 
be  partially  obviated  by  a  more  rigorous  application  of  the 
method. 

Thus  in  making  the  tests  for  the  just  observable  differ- 
ences above  and  below,  weights  must  first  be  taken  that  are 
not  recognizably  different  from  the  standard,  and  must  then 
be  slowly  increased  or  decreased  till  just  observably  different. 
Subjective  equality  must  be  regarded  rather  than  objective 


16  LABORATORY   COURSE  IN  PSYCHOLOGY. 

equality,  if  the  two  are  at  odds,  as  sometimes  happens.  To 
these  tests  two  others  must  be  added ;  namely,  for  the  just 
tmobservable  differences  above  and  below,  the  operator  now 
selecting  a  weight  that  is  clearly  heavier  than  the  standard 
and  decreasing  it  gradually  till  it  can  just  no  longer  be 
recognized  as  different,  and  similarly  selecting  one  that  is  at 
first  clearly  lighter  than  the  standard  and  increasing  it  till 
it  seems  the  same.  The  average  of  the  four  tests,  just 
recognizably  different  and  just  -^^recognizably  different,  is 
then  taken  for  the  ratio.  When  great  accuracy  is  required 
the  method  must  be  used  in  this  complete  form.  For  other 
methods  and  fuller  literature,  see  the  chapter  on  Weber's 
Law  below. 

Weber,  543-549;  Wundt,  3te  Aufl.,  I.,  343  ff.,  350;  4te  Aufl.,  L, 
336  f.,  341  ff. 

25.  Temperature  and  Pressure.     Cold  and  hot  bodies  feel 
heavier  than  bodies  of  equal  weight  at  a  normal  temperature. 

a.  For  cold,  take  two  dollar  pieces,  warm  one  until  it 
ceases  to  seem  cold ;  cool  the  other  to  10°  C.     Apply  alter- 
nately to  the  palm  of  the  hand,  letting  the  hand  rest,  mean- 
while, on  the  table  or  some  other  support  so  as  to  exclude 
"muscle  sense."     The   cold  one  will  seem  much  heavier, 
perhaps  as  heavy  as  two  at  the  normal  temperature.     The 
same  experiment  may  be  tried  on  the  forehead  with  the  head 
supported. 

b.  For  heat  take  two  wooden  cylinders  of  equal  weight ; 
heat  one  to  a  high  temperature  by  standing  it  on  end  in  a 
metal  vessel  floating  in  a  water  bath.     Apply  the  cylinders 
on  end  alternately  to  the  back  of  the  hand  (supported)  be- 
tween the  metacarpal  bones  of  the  thumb  and  first  finger. 
The  hot  one  will  seem  heavier. 

Weber,  512,  551;  Szabadfoeldi ;  Funke,  320;  Dessoir,  304-306. 

26.  Pressure  Evenly  Distributed  over  a  Considerable  Area 
is  less  strongly  felt  than  pressure  upon  an  area  bordered  by 


THE  DERMAL  SENSES.  17 

one  that  is  not  pressed.  Dip  the  hand  up  to  the  wrist  into 
water  (or,  better  still,  into  mercury)  of  normal  temperature, 
and  notice  that  the  sensation  of  pressure  is  strongest  in  a 
ring  about  the  wrist  at  the  surface  of  the  water ;  possibly 
stronger  on  the  volar  than  on  the  dorsal  side.  The  ring 
effect  is  unmistakable  when  the  hand  is  moved  up  and  down 
in  the  water. 

27.  Pressures  are  not  Equally  well  Perceived  in  all  Parts 
of  the  Body.     This  may  be  tested  with  weights  applied  some- 
what as  in  Ex.  24,  as  was  done  by  Weber,   but  a  simpler 
experiment  may  be  made  as  follows :  Find  the  pulse  at  the 
wrist ;  feel  it  with  the  finger  tips,  the  back  of  the  fingers, 
the  side  of  the  hand,  the  other  wrist,  the  lip,  and  the  tip  of 
the  tongue.     Try  the  pulse  in  the  temple  with  the  finger 
tips,  the  side  of  the  hand,  and  the  fore-arm.     Notice  that 
when  it  is  felt  by  another  person  the  experimenter  is  unable 
to  feel  it  subjectively. 

Goltz. 

28.  Eefinement  of  Active  Pressure  Sense.     Something  of 
the  refinement  of  the  pressure  sense  in  perceiving  the  un- 
evenness  of  surfaces  may  be  found  by  laying  a  hair  on  a 
plate  of  glass  or  other  hard,  smooth  surface  and  over  it  10 
or  15   sheets  of  writing-paper.     The  position  of  the   hair 
can  easily  be  felt  by  passing  the  finger  tips  back  and  forth 
over  the  paper. 

29.  The   Hairs   as   Organs  of  Touch.     The  finest  hairs 
respond  with   a  distinct   sensation   of  anticipatory  touch, 
when  they  are  moved,  and  probably  this  accounts  for  a  part 
at  least  of  the  differences  between  the  fore-arm  and  finger 
tips  found  in  Ex.  22.     Touch  a  few  single  hairs  and  observe 
the  sensation. 

Blaschko. 

30.  The  Feeling  of  Traction  or  Negative  Pressure  has 


18  LABORATORY  COURSE  IN  PSYCHOLOGY. 

been  discriminated  by  some  authors,  but  has  rarely  been 
made  an  object  of  experiment.  It  is  to  be  observed  when 
viscid  substances  are  handled,  when  a  portion  of  the  skin  is 
brought  over  the  mouth  of  a  closed  vessel  and  the  air  ex- 
hausted, or  when  in  any  other  way  the  skin  is  lifted  from 
the  underlying  portions  of  a  member.  The  sensation  may 
be  studied  qualitatively  by  passing  a  thread  through  a 
small  bit  of  court-plaster,  knotting  it  on.  the  gummed  side 
and  sticking  the  plaster  to  the  skin.  Traction  on  the  thread 
now  produces  the  sensation. 
Hall  and  Motora,  93  ff. ;  Bloch. 

GENERAL  SENSATIONS,  TICKLE,  AND  PAIN. 

These  topics,  though  clearly  of  very  great  psychological 
interest,  have  so  far  received  comparatively  little  careful 
study,  and  few  experiments  have  been  made  upon  them. 
They  are  not  exclusively  dermal  senses,  but  the  skin  offers 
the  most  convenient  field  for  the  study  of  the  two  to  be 
considered  here,  namely,  tickle  and  pain.  In  both  the 
experimenter  should  notice  the  subjective  cast  of  the  sensa- 
tions. Our  eyes  and  ears  give  us  information  about  colored 
and  sounding  things,  but  tickle  and  pain  let  us  know  that 
we  are  being  tickled  or  hurt  by  something. 

31.  Tickle.  Two  sorts  of  tickle  are  easily  distinguish- 
able, a  deep-seated  tickle  located  in  the  rib  region,  which 
seems  more  strongly  developed  in  children,  and  responds  to 
rather  strong  stimulation,  and  a  superficial  tickle  much 
more  widely  distributed,  and  responding  to  slight  stimuli 
only.  The  latter  sort  is  that  regarded  in  this  group  of 
experiments. 

a.  Touch  very  lightly  the  different  parts  of  the  face,  es- 
pecially about  the  eyes,  the  margin  of  the  lips  and  the 
opening  of  the  ears  with  the  tip  of  a  light  wisp  of  paper 
and  notice  the  tickle  sensations.  Notice  the  apparent 


THE  DERMAL  SENSES.  19 

disproportion  between  the  stimulus  and  the  resulting  sen- 
sation, the  wide  and  indefinite  irradiation,  and  the  long 
after-image.. 

b.  Touch  the  same  parts  as  lightly  as  possible  with  the 
tip  of  a  penholder  or  the  finger,  and  then  with  the  same 
instrument  while  exerting  at  the  same  time  a  moderate  pres- 
sure.    Notice  the  difference  in  effect ;  notice  also  that  the 
tendency  to  rub  a  tickled  surface  is  a  tendency  to  use  a 
greater  stimulus  to  remove  the  effects  of  the  less.     Notice 
also,  when  feeling   a  tendency  to  sneeze,  that  the  sneeze 
can  be  wholly  prevented  by  firm  pressure  or  rubbing  of  the 
sides  of  the  nose  or  the  adjacent  parts  of  the  face. 

c.  Tickle  is  apparently  a  summation  phenomenon.    Touch 
the  tip  of  the  tongue  lightly  with  the  prong  of  a  tuning- 
fork  at  rest  and  notice  the  after-image,  which,  however,  has 
no  tickle  in  it.     Then  strike  the  fork  and  touch  it  to  the  tip 
of  the  tongue.     Compare  the  effects. 

d.  The  ticklability  of  adjacent  parts  of  the  body  is  quite 
markedly  different.     Test  with  the  tuning-fork,  striking  it 
and  applying  it  gently  to  the  tip,  sides,  and  middle  of  the 
upper  surface  of  the  tongue  and  to  the  lower  surface. 

32.  Pain.  a.  Slow  conduction.  Eemove  the  shoe  and 
strike  a  smart  blow  with  a  light  rod  on  the  sole  of  the  foot, 
or  on  a  corn ;  the  pain  will  be  perceived  noticeably  later  than 
the  first  sensation  of  contact,  separated  from  it  perhaps  by 
an  almost  empty  interval.  This  delay  is  probably  due  to 
the  same  cause  as  the  secondary  after-image  of  touch  in 
Ex.  11. 

b.  Temperature  pains.  A  given  increase  of  heat  above 
the  blood  temperature  is  more  effective  in  causing  pain  than 
an  equal  decrease.  Compare  the  effects  of  plunging  the 
hand  into  water  at  10°C.  and  at  60°.  Use  a  considerable 
quantity  of  water  and  do  not  allow  the  hand  to  remain  too 


20         LABORATORY  COURSE  IN  PSYCHOLOGY. 

long  in  the  water,  for  its  sensibility  to  pain  as  well  as  to 
temperature  is  decreased  by  fatigue. 

Experiments  on  pain  can  likewise  be  made  with  electrical 
stimulation  and  pressure.  These  are  especially  suitable  for 
determining  the  relative  sensibility  of  different  subjects. 
The  first  can  easily  be  tried  with  the  sliding  induction  coil, 
by  applying  the  electrodes  to  the  surface  to  be  tested  and 
then  gradually  pushing  the  secondary  coil  towards  the 
primary  till  the  stimulation  becomes  painful.  For  appara- 
tus, see  the  chapter  on  apparatus  below. 

Weber,  569  ff. ;  Dessoir,  Beaunis,  Lombroso,  Mantegazza,  Preyer,  89. 


BIBLIOGKAPHY. 

IN  the  following  bibliography,  and  those  appended  to  later  chapters, 
the  aim  has  not  been  to  make  an  exhaustive  list,  but  rather  to  give 
a  number  of  the  more  important  references  by  which  the  student 
may  begin  the  study  of  original  sources  if  he  desires.  For  the  same 
reason  the  list  has  not  been  kept  strictly  to  works  where  the  experi- 
ments of  the  chapter  are  discussed,  but  a  few  other  important  refer- 
ences have  been  given.  If  any  one  wishes  to  increase  the  list,  he 
can  easily  do  so  from  the  reviews  in  the  various  philosophical  and 
psychological  journals,  and  from  the  classified  bibliography  on  psy- 
chology and  the  physiology  of  the  sense  organs  published  yearly  in 
the  Zeitschrift  fur  Psychologic,  beginning  with  the  literature  of 
1889.  For  the  older  literature  the  rich  citations  of  Volkmann's 
Lehrbuch  der  Psychologic  may  be  consulted.  Much  psychological 
literature  appears  at  present  in  the  physiological  periodicals.  The 
Centralblatt  fur  Physiologic  contains  reviews  and  an  annual  bibli- 
ography of  general  physiology,  with  sections  on  the  physiology  of  the 
senses  and  physiological  psychology,  and  has  done  so  since  its  begin- 
ning, in  1887.  Hermann's  Handbuch  der  Physiologic  makes  many 
references  to  literature,  and  each  important  section  in  Beaunis' s 
Elements  de  Physiologic  humaine  is  followed  by  a  bibliography. 
Hoffmann  and  Schwalbe's  Jahresberichte  iiber  die  Fortschritte  der 


THE  DERMAL   SEN 


Anatomie  und  Physiologic  gives  bibliographies  and  summaries  of 
literature  since  1872.  The  Index  Medicus,  now  in  its  fifteenth 
volume,  has  listed  all  current  medical  literature  since  1879  ;  and  in 
connection  Vith  this  may  be  mentioned  the  mammoth  Index  Cata- 
logue of  the  Library  of  the  Surgeon-General's  Office,  United  States 
Army,  of  which  thirteen  large  volumes  have  so  far  been  published 
(September,  1893),  extending  from  A  to  Sutugin.  The  descriptive 
pamphlet  of  the  Harvard  Psychological  Laboratory  also  contains, 
in  a  ten-page  appendix,  a  classified  list  of  psychological  literature. 

ARISTOTLE  :  nepl  Ei/v^W,  c.  2,  Bekker,  460.     Also  in  German  trans- 

lation by  Johannes  Miiller,  as  an  appendix  to  his  Ueber  die 

phantastischen  Gesichtserscheinungen.     Coblenz,  1826. 
AUBERT  UND  KAMMLER:  MoleschotVs  Untersuchungen,  V.,  1859V 

145. 

BEAUNIS  :  Les  Sensations  internes,  Paris,  1889. 
BLASCHKO  :  Zur  Lehre  von  den  Druckempfindungen,  Verhandl.  d. 

Berliner  physiol.  Gesell.,  Sitz.  27  Marz,  1885.     Du  Bois-Rey- 

mond's  Archiv,  1885,  349. 
BLIX  :  Experimented  Beitrage   zur  Losung  der   Frage   iiber   die 

specifische  Energie  der  Hautnerven,  Zeitschrift  fur  Biologic, 

XX.,  1884,  141-156. 
BLOCH:  Kecherches  expe"rimentales  sur  les  sensations  de  traction 

et  de  pression  cutanSes,  Archives  de  Physiologic,  Ser.  5,  III., 

1891,  322-333. 
BRONSON  :   The  Sensation  of  Itching,  Medical  Record,  XXXVIII., 

1890,  No.  1041,  Oct.  18,  425-429. 

DESSOIR:  Ueber  den  Hautsinn,  Du  Bios-Reymond"1  s  Archiv,  1892, 
175-339.  See  review  of  this  paper  by  Goldscheider,  Zeitschrift 
fur  Psychologic,  V.,  1893,  117-122. 

DONALDSON:  On  the  Temperature-sense,  Mind,  X.,  1885,  399-416. 

Du  BOIS-REYMOND,  EENE  :  Ueber  chemische  Reizung  des  Tempera- 
tursinnes.  Verhandlungen  der  Berliner  physiol.  Gesellsch., 
Sitz.  11  Nov.  1892.  Du  Bois-Reymond's  Archiv.,  1893,  187-190. 

FECHNER:  Elemente  der  Psychophysik,  L,  201-211  (temperature). 

FUNKE:  Der  Tastsinn  und  die  Gememgefiihle,  Hermann's  Hand- 
buch  der  Physiol.,  Vol.  III.,  pt.  2,  289-414. 


22         LABORATORY  COURSE  IN  PSYCHOLOGY. 

GOLDSCHEIDER:  A.  Ueber  Warme-,  Kalte-  und  Druckpunkte, 
Verhandl.  d.  Berliner  physiol.  Gesell.,  Sitz.  13  Marz,  1885. 
Du  Bois-Reymond's  Archiv,  1885,  340-345. 

B.  Neue  Thatsachen   tiber  die    Hautsinnesnerven.   Ibid.   1885, 
Supplement  Band,  1-110,  5  plates. 

C.  Zur  Dualitat  des  Tempera  tursinns,  Pflilger's  Archiv,  XXXIX., 

1886,  96-120.     (On  Herzen's  experiments.) 

D.  Ueber  die  specifische  Wirkung  des  Menthols  auf  die  Tempera- 
tur-Nerven,    Yerh.  d.  Berliner  physiol.  Gesell.,  9  April,  1886. 
Du  Bois-Reymond's  Archiv,  1886,  555. 

E.  »Histologische  Untersuchungen  tiber  die    Endigungsweise  der 
Hautsinnesnerven  beim  Menschen.     Ibid.   1886,  Supplement- 
Band,  191-231. 

F.  Die  Einwirkung  der  Kohlensaure  auf  die  sensiblen  Nerven  der 
Haut,  Verh.  d.  Berliner  physiol.  Gesell.,  4  Nov.   1887.     Ibid. 

1887,  575-580. 

G.  Ueber  die  Topographic  des  Temperatursinns,  Yerh.  d.  Ber- 
liner physiol.  Gesell.,  Sitz.  1  Juli,  1887.     Ibid.  1887,  473-476. 

H.  Ueber  die  Summation  von  Hautreizen,  Yerhandlungen  der  phy- 
siol. Gesellsch.  zu  Berlin,  Sitz.  31  Oct.  1890.  Ibid.  1891,  164- 
169. 

L  Die  Lehre  von  den  specifischen  Energieen  der  Sinnesnerven, 
Berlin,  1881.  A  forty  page  dissertation,  containing  full  refer- 
ences to  literature. 

GOLTZ  :  Ein  neues  Yerf ahren  die  Scharf  e  des  Drucksinns  der  Haut 
zuprufen,  Centralblatt  fur  med.  Wiss.,  1863,  No.  18,  273-276. 

HALL  AND  DONALDSON:  Motor  Sensations  of  the  Skin,  Mind,  X., 
1885,  557-572. 

HALL  AND  MOTORA:  Dermal  Sensitiveness  to  Gradual  Pressure 
Changes,  American  Journal  of  Psychology,  I.,  1887,  72-98. 

BERING:  Der  Temperatursinn,  Hermann's  Handbuch  der  Physi- 
ologic, Yol.  III.,  pt.  2,  415-439. 

HERZEN  :  Ueber  die  Spaltung  der  Temperatursinnes  in  zwei  geson- 
derte  Sinne,  Pflilger's  Archiv,  XXXYIIL,  1886,  93-103. 

HOPPE  :  Das  Aristotelische  Rathsel  der  mit  den  gekreuzten  Finger- 
spitzen  gefuhlten  Kugel,  Wiener  med.  Presse,  1888,  Nos.  22,  23, 
785,  827. 


THE  DERMAL    SENSES.  23 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 

LADD  :  Elements  of  Physiological  Psychology,  New  York,  1887. 

LOEB:   Untersuchungen  tiber  den  Fiihlraum  der  Hand,  Pfluger's 

Archiv,  XLL,  1887,  107-127. 
LOMBBOSO  :  Algometria  elettrica  nel  uomo  sano  e  alienato,  Milano, 

1867. 
(Lombroso  und  Ottolenghi)  Die  Sinne  der  Yerbrecher,  Zeitschrift 

fiir  Psychologic,  II.,  1891,  337  ff. 
LOTZE  :  A.  Medicinische  Psychologic,  Leipzig,  1852. 
B.   Outlines  of  Psychology,  translated  by  Herrick,  Minneapolis; 

also  by  Ladd,  Boston,  1885. 

MANTEGAZZA:  La  Physiologic  de  la  Douleur,  Paris,  1888. 
PBEYER  :  Ueber  den  Farben-  und  Temperatur-Sinn  mit  besonderer 

Kucksicht  auf  Farbenblindheit,  Pfluger's  Archiv,  XXY.,  1881, 

especially  pp.  75-92. 
QUINCKE  :  Ueber  Mitempfindungen  und  verwandte  Yorgange,  Zeitsch. 

fur  klin.  Medicin,  XYIL,  1890. 
SCBTWANEB  i  Die  Priifung  der  Hautsensibilitat  vermittelst  Stimm- 

gabeln  bei   Gesunden  und   Kranken,  Inaug.  Diss.,  Marburg, 

1890,   37.     Keview  with    table  of    sensibility,   Zeitschrift  fiir 

Psychologic,  II.,  1891,  398. 
SEBGI:  Su  alcuni  caratteri  del  senso  tattile,  Eivista  di  Filosojia 

Scientiftca,  1891.     Same  paper  in  German,  Zeitschrift  fur  Psy- 
chologic, III.,  1892,  175-184. 
STUMPF:  Zum  Begriff  der  Lokalzeichen,  Zeitschrift  fur  Psychologic, 

IY.,  1892,  70-73. 

SZABADFOELDI:  Molcschott' s  Untersuchungen,  IX.,  1865,  631. 
YIEBOBDT:  Physiologic  des  Menschen,  Tubingen,  1877,  340  ff. 

WEBEB:  Der  Tastsinn  und  das  Gemeingefiihl,  Wagner's  Handwor- 

terbuch  der  Physiologic,  III.,  2,  481-588. 
Yo^r  WITTICH:    Bemerkungen  zu  Preyers  Abhandlung    iiber  die 

Grenzen  des  Empfindungsvermogens    und  Willens,  Pfluger's 

Archiv,  II.,  1869,  329-350. 
WUNDT:    Grundziige  der  physiologischen  Psychologic,   3te  Aufl., 

Leipzig,  1887,  I.,  391  ff.,  II.,  5   ff.;  4te  Aufl.,  Leipzig,  1893, 

I.,  410  ff. 


24         LABORATORY  COURSE  IN  PSYCHOLOGY. 

For  further  bibliographical  references  see  especially  the  citations  of 
the  following  authors:  On  Touch,  Dessoir  and  Goldscheider. 
On  Temperature,  Dessoir,  Donaldson,  Goldscheider.  On  Pres- 
sure, see  bibliographies  following  the  chapter  on  Weber's  Law 
and  the  account  of  pressure  sense  apparatus  below.  On  Pain 
in  general,  see  Bain,  Mind,  Ser.  2,  I.,  1892,  161;  Marshall, 
Ibid.  Ser.  1,  XIV.,  1889,  511;  XYL,  1891,  327,  470;  Ser.  2,  L, 
1892,358,  453;  Philosophical  Review,  I.,  1892,  625;  Nichols, 
Ibid.  L,  1892,  403,  518. 


KIN  ESTHETIC  AND   STATIC  SENSES.  25 


CHAPTER    II. 
Kinaesthetic  and  Static  Senses. 

THIS  group  of  senses  furnishes  us  data  for  the  perception 
of  the  positions  and  motions  of  our  members  and  of  the 
body  as  a  whole,  and  plays  a  leading  part  in  the  perception 
of  space.  It  includes  some  senses  whose  existence  or 
efficiency  is  disputed  (Innervation  Sense l  and  Muscle  Sense), 
and  others  whose  independence  has  only  of  late  been  gener- 
ally recognized  (Joint  Sense  and  Tendon  Sense).  All  are 
closely  united  with  one  another  and  with  pressure  and 
contact,  and  some  are  hardly  ever  dissociated  except  by 
disease.  This  chapter  is  necessarily  limited  to  the  experi- 
mental side  of  the  subject  and  to  the  simpler  experiments 
to  be  found  there.  Many  of  the  most  important  psycho- 

1  The  term  "  innervation  sense  "  must  not  be  taken  too  strictly  as  meaning  a 
wholly  independent  sense  of  motor  discharge,  as  it  has  often  been  taken.  Says 
Wundt,  in  his  last  edition  (4te  Aufl.  I.,  425):  "Manifold  observations  make  it 
probable  that  the  central  components  of  the  sensations  accompanying  active 
movements  have  their  origin  in  the  memory-images  of  movements  previously 
executed,  which  partly  initiate,  partly  accompany,  each  voluntary  movement. 
Since  memory-images  possess  qualitatively  the  same  sensory  content  as  the 
original  perceptions,  such  central  sensations  of  effort  and  movement  (Kraft-  und 
Bewegungsempfindungeri)  will  under  normal  conditions  blend  completely  with  the 
more  intense  peripheral  sensations  of  the  same  kind;  they  will,  however,  produce 
an  independent  effect,  if  from  any  cause  the  peripheral  sensations  fall  away.  It 
would  be  proper,  therefore,  to  give  up  the  term  "  innervation  sensations  "  for  the 
sensations  in  question ;  because  it  is  liable  to  convey  the  false  impression  that 
these  are  sensations  which  in  and  for  themselves,  without  any  relation  to  the 
peripheral  components  of  the  sensations  of  effort  and  movement,  accompany 
the  motor  innervation.  This  assumption,  which  as  a  rule  has  formerly  been  con- 
nected with  the  notion  of  "innervation  sensations,"  is,  however,  very  improb- 
able." Cf.  also  p.  431,  and  in  the  third  edition  I.,  p.  405  ff. 


26          LABORATORY  COURSE  IN  PSYCHOLOGY. 

logical  problems  involve  the  motor  sensations  of  the  eye, 
some  of  which  are  considered  in  Chap.  VII. 

MUSCLE  SENSE,  Kraftsinn. 

Whether  there  are  any  specific  muscular  sensations 
distinct  from  those  that  come  from  other  parts  of  the 
member  in  motion  cannot  now  be  asserted  with  positive- 
ness  ;  but  even  if  there  be  such,  the  part  that  they  play  in 
our  ordinary  motor  perceptions  is  probably  a  minor  one. 
The  term  "  muscle  sense,"  however,  has  been  used  to  desig- 
nate the  whole  group  of  motor  sensations,  and  is  here  re- 
tained for  that  purpose. 

33.  Lifted  Weights.      a.  Weights    lifted    slowly   seem 
heavier  than  the  same  weights  lifted  rapidly.     Lift  the 
same  weight  twice,  lifting  it  first  at  the  most  natural  and 
convenient  rate,  and  the  second  time  very  slowly,  beginning 
with   much  less   than   the  necessary  effort  and   gradually 
increasing  it  till  the  weight  rises. 

b.  Lift  a  moderate  weight  with  one  hand  and  at  the  same 
time   clench   the   other    sharply.     The   weight   will   seem 
lighter  than  when  no  simultaneous  effort  is  made. 

c.  Repeat  Ex.  23,  using  active  lifting  instead  of  pressure 
in  comparing  the  weights. 

Charpentier ;  on  a,  Goldscheider,  A,  186. 

34.  Discriminative  Sensibility  for  Lifted  Weights. 

a.  Find  by  the  method  of  experiment  used  in  Ex.  24  what 
is  the  just  observable  difference  above  and  below  a  standard 
weight  of  100  grams,  when  the  weights  are  lifted  instead  of 
merely  being  allowed  to  press  upon  the  skin.  In  this  ex- 
periment lift  the  weights  successively  with  the  same  hand. 
The  weights  must  be  placed  near  together  within  convenient 
reach,  and  care  must  be  taken  that  both  are  lifted  at  the 
same  rate  and  to  the  same  height.  Let  the  subject  lift  one 


KIN^ESTKETIC  AND   STATIC  SENSES.  27 

weight  and  then  the  other,  and  render  his  decision  after 
once  lifting  each.  In  half  of  the  trials  let  the  standard 
weight  be  placed  at  the  left  side  of  the  weight  to  be  com- 
pared and  be  lifted  first ;  in  the  other  half  let  the  weight  to 
be  compared  stand  at  the  left  and  lead  in  the  lifting. 

b.  Eepeat  the  experiment,  letting  the  subject  lift  the 
standard  with  one  hand,  and  the  comparison  weight  with 
the  other,  keeping  the  same  hand  for  each  during  each  set 
of  trials  (that  is,  during  a  determination  of  the  just  ob- 
servable difference  above  and  below),  but  combining  a  num- 
ber of  sets  with  the  standard  in  the  right  hand  with  an 
equal  number  in  which  it  is  in  the  left.  Find  also  from  the 
figures  the  ratios  when  the  standard  is  in  the  right  hand 
and  when  it  is  in  the  left  hand,  for  use  in  Ex.  35.  Com- 
pare the  ratios  found  in  these  experiments  with  that  found 
in  Ex.  24. 

In  these  experiments  the  sense  of  pressure  might  be  ex- 
pected to  co-operate ;  but  when  it  is  excluded,  or  put  at  a 
relative  disadvantage,  the  sensibility  for  differences  of  lifted 
weights  is  not  diminished.  Weber's  method  of  excluding 
the  pressure  sense  was  to  wrap  the  weights  in  pieces  of 
cloth,  and  lift  them  by  the  four  corners  together.  The 
pressure  on  these  corners  can  be  changed  at  will,  irrespec- 
tive of  the  heaviness  of  the  weight  lifted. 

For  fuller  literature  on  lifted  weights,  see  the  chapter  on 
Weber's  Law  below. 

Weber,  546-547;  Miiller  und  Schumann;  James,  II. ,  189  ff.,  486  ff. ; 
Beaunis;  Wundt;  Fullerton  and  Cattell. 

35.  Adjustment  of  the  Motor  Discharge.  After  having 
performed  the  second  part  of  Ex.  34,  compare  the  standard 
weight  with  a  very  much  heavier  weight,  e.g.,  2  kg.,  with 
all  the  circumstances  of  actual  careful  judgment.  Practise 
this  judgment  thirty  times,  leaving  a  longer  time  between 


28         LABOEATOEY  COUESE  IN  PSYCHOLOGY. 

the  individual  comparisons  than  between  liftings  of  the 
weights  compared.  Then  at  once  return  to  the  smaller 
weights,  giving  the  standard  to  the  same  hand  as  before, 
and  to  the  hand  that  has  just  been  lifting  the  2  kg.  the 
weight  to  be  compared.  Not  only  will  the  weight  just  rec- 
ognizably heavier  before  seem  considerably  lighter  than  the 
standard,  but  also  still  heavier  weights  will  seem  so.  This 
time  the  tests  must  be  few,  not  more  than  three  or  four.  If 
more  tests  are  desired,  practise  the  comparison  of  the  stand- 
ard and  2  kg.  weight  again  ten  times  before  taking  them. 
By  the  practice  the  nervous  centres  discharging  into  the 
muscles  that  raise  the  2  kg.  weight  become  accustomed  to 
a  larger  discharge  than  that  required  for  the  small  weights 
and  do  not  at  once  re-adapt  themselves,  but  supply  too  great 
a  discharge.  The  weight  now  rises  with  greater  rapidity 
than  the  standard,  and  is  consequently  pronounced  lighter 
(Miiller  and  Schumann),  or  the  balance  between  the  ex- 
tensors and  flexors  that  was  suited  to  raising  the  heavier 
weight  is  not  suited  to  the  lighter  weight,  and  the  second  is 
pronounced  lighter  because  of  the  strain  in  the  extensors 
necessary  to  restore  the  balance  (Delabarre).  This  experi- 
ment seems  conclusive  against  a  well-developed  and  inde- 
pendent innervation  sense ;  for  if  there  were  any  sensation 
of  nervous  discharge,  we  ought  to  know  when  we  go  from 
a  very  heavy  to  a  light  weight  that  the  discharge  is  dis- 
proportionate ;  but  we  do  not. 

Miiller  und  Schumann;  but  cf.  also  Fullerton  and  Cattell,  131, 
and  Delabarre. 

INNERVATION  SENSE, 

36.  Simultaneous  Movements.  The  evidence  most  fre- 
quently offered  in  support  of  a  special  innervation  sense  is 
clinical  and  therefore  beyond  the  scope  of  this  course.  Ex- 
periments of  the  type  of  the  following  have  been  brought 
forward,  but  their  interpretation  has  been  disputed. 


KIN^STUETIC  AND    STATIC  SENSES.  29 

a.  Stand  erect  before  the  blackboard,  with  the  eyes  closed 
and  coat  off,  if  it  interferes  with  free  motion  of  the  arms. 
Draw  with  jeach  hand,  using  both  at  once,  a  conventional  leaf 
pattern  like  those  in  the  annexed  cut,  drawing  always  from 
a  to  b.     In  drawing,  try  to  make  the 

lobes  of  the  leaf  of  equal  size,  like 
those  in  Fig.  1 ;  draw  each  with  a 
single  simultaneous  "  free-hand  "  mo- 
tion  of  the  arms,  that  is,  draw  each 
with  a  single  volitional  impulse  di- 
rected equally  to  the  two  sides ;  the 
last  point  is  important.  First  draw 
a  pair  of  leaves,  beginning  them  with 
the  hands  before  the  shoulders  at  the 
same  height;  the  result  will  be  ap-  \  Fig. 2. 

proximately  like  Fig.  1.      Next  draw 
a  pair  with  one  hand  about  a  foot  higher  than  before,  the 
other  about  a  foot  lower  ;  the  result  will  be  like  Fig.  2. 

b.  Bring  the  hands  again  to  the  position  used  in  drawing 
Fig.  1,  and  draw  a  pair  of  leaves  having  their  apices  right 
and  left.     The   leaves  will   be   symmetrical.     Next  begin 
with  one  hand  about  a  foot  farther  away  from  the  median 
plane  than  before  and  the  other  at  it,  but  both  at  the  same 
level.     Draw  as  before  ;  asymmetrical  leaves  will  be  the  re- 
sult.    Repeat  the  drawing  a  number  of  times,  sometimes  rais- 
ing or  extending  one  arm,  sometimes  the  other.     In  general 
it  will  be  found  that,  notwithstanding  the  intention  to  make 
equal  movements  of  the  hands,  the  motions  of  further  ex- 
tension in  the  extended  arm  and  of  further  flexion  in  the 
flexed  arm  are  too  short,  and  those  in  the  contrary  direction 
in  each  case  too  long.     The  argument  founded  on  this  ex- 
periment runs  as  follows  :  We  think  that  our  hands  execute 
equal  movements,  when  they  do  not,  because  we  are  con- 
scious of  willing  equal  movements,  and  unconscious,  or  only 


30          LABORATORY  COURSE  IN  PSYCHOLOGY. 

inexactly  conscious,  of  those  actually  made.  If  we  per- 
ceived motion  of  our  members  by  the  skin,  joint,  and  muscle 
sensations  that  accompany  their  motion  (as  the  opponents 
of  the  innervation  sense  believe)  we  ought  to  know  the  ex- 
tent to  which  our  hands  are  moved  each  time,  and  not  to 
fall  into  the  illusion  that  we  find  in  these  experiments. 
Cf.  Ex.  44  d. 
Loeb,  B,  15  ff. 

37.  Illusory  Movement  in  an  Immovable  Member..    Lay 
the  hand  palm  downward  on  the  edge  of  the  table  or  on  a 
thick  book  so  that  the  last  three  fingers  shall  be  supported 
and  held  extended  while  the  thumb  and  first  finger  remain 
free.     Bend  the  first  finger  considerably  at  both  the  inner 
joints,  and  hold  it  in  position  with  the  other  hand.     The 
finger-tip  is  still  movable,  as  will  be  found  on  touching  it ; 
but  it  is  anatomically  impossible  to  move  it  voluntarily. 
When,  however,  the  effort  is  made  to  move  it  (the  eyes  be- 
ing closed),  there  is  a  sensation  of  motion,  though  no  actual 
motion  is  possible.     From  this,  an  inner  sense  of  motion 
(innervation   sense)   has   been   inferred.     When   operating 
upon  another  subject,  the  operator  may  hold  the  finger  in 
position,  and  require  the  subject  to  execute  with  the  corre- 
sponding finger  of  his  free  hand  a  motion  equal  to  that 
which  he  thinks  he  makes  with  the  one  that  is  held.     Ob- 
serve, however,  that  the  tendons  in  the  wrist  move,  and 
that  there  are  slight  movements  elsewhere  in  the  hand. 

Sternberg;  James,  II.,  105,  515,  footnote;  Goldscheider,  A,  317. 

38.  Terrier's  Experiment.     That  the  feeling  of  effort  is 
largely,  if  not  entirely,  of  peripheral  rather  than  central  ori- 
gin, appears  from  such  experiments  as  the  following.     Hold 
the  finger  as  if  to  pull  the  trigger  of  a  pistol.     Think  vigor- 
ously of  bending  the  finger,  but  do  not  bend  it  j  an  unmis- 


KIN  AESTHETIC  AND   STATIC  SENSES.  31 

takable  feeling  of  effort  results.  Eepeat  the  experiment, 
and  notice  that  the  breath  is  involuntarily  held,  and  that 
there  are  tensions  in  other  muscles  than  those  that  would 
move  the  finger.  Eepeat  the  experiment  again,  taking  care 
to  keep  the  breathing  regular  and  the  other  muscles  passive. 
Little  or  no  feeling  of  effort  will  now  accompany  the  imagi- 
nary bending  of  the  finger. 

Ferrier,  382  ff.  (English  Ed.). 

On  Innervation  Sense  in  general,  besides  the  authors  already  men- 
tioned, see  :  Wundt,  3te  Ann.  I.,  397  ff.,  4te  Ann.  I.,  423  ff.;  James, 
II.,  486  ff. ;  Goldscheider,  A,  206  ff. 

SENSATIONS  OF  MOTION,  JOINT  SENSATIONS. 

39.  Passive  Motion  at  the  Elbow.  Let  the  subject  rest 
his  fore-arm  flat  upon  the  arm-board  of  the  instrument 
(bringing  his  elbow 
over  the  hinge),  and 
close  his  eyes.  Let 
the  operator  then  raise 
or  lower  the  free  end 
of  the  arm-board 
slowly  by  pressing 
down  or  lifting  the 
counter  weight,  and 
require  the  subject  to 
announce  when  he  first 
perceives  the  motion 
of  his  fore-arm.  Re- 
cord the  angular  move- 
ment required  to  produce  a  just  observable  sensation. 
Notice  that  the  movement  seems  to  be  located  chiefly  in  the 
hand.  It  is  extremely  important  not  to  mistake  the  sensa- 
tion of  increased  pressure  or  of  jar  for  that  of  motion.  The 
rate  of  movement  will  be  found  important,  and  should  be 


32          LABOEATOEY  COUESE  IN  PSYCHOLOGY. 

kept  as  constant  as  possible.     The  results  found  in  this 
way  are  rough  ;  for  more  exact  methods  see  Goldscheider,  A. 

40.  Active  Movement  of  the  Last  Joint  of  the  Finger. 
The  joint  sensations  of  the  fingers  are  less  fine  than  those 
of  the  elbow,  but  are  more  convenient  for  demonstration  of 
active  flexion.     Fasten  a  piece  of  straw,  with  court-plaster 
or  otherwise,  to  the  finger-nail  of  the  middle  finger,  and  cut 
it  off  at  such  a  length  that  the  distance  from  the  joint  of 
the  finger  to  the  end  of  the  straw  shall  be  115  mm.     With 
that  radius  2  mm.  corresponds  to  about  1°  of  angular  meas- 
ure.    Rest  the  hand  on  a  thick  book,  letting  the  last  joint  of 
the  finger  extend  beyond  the  edge.     Set  up  a  millimeter 
scale  at  right  angles  with  the  straw.     Close  the  eyes  and 
make  the  least  possible  flexion  of  the  finger  at  the  last  joint, 
having  an  assistant  note  its  extent  on  the  scale.     Close  at- 
tention may  perhaps  be  able  in  both  the  active  and  passive 
movements  to  locate  the  sensation  in  the  joint,  but  more 
rigorous    experiments  are  required   to  show  its  character 
clearly,  and  to  prove  its  location. 

Goldscheider,  A. 

41.  Location  of  Movements,     a.  Motions  on  the  skin  can 
be  interpreted  either  as  the  movement  of  an  object  over  the 
surface  of  the  skin,  or  of  the  skin  over  the  surface  of  the 
object.     This  opens  the  way  for  illusions.     Have  an  assis- 
tant draw  a  pencil-point  gently  across  the  wrist  or  the  finger- 
tips of  the  observer,  who  sits  with  closed  eyes.     A  tendency 
to  interpret  the  sensation  as  motion  of  the  wrist  or  finger 
will  be  observed.     The  hand  and  arm  must  be  held  free,  so 
that  the  illusion  may  not  be  corrected  by  the  presence  of 
other  touch  sensations. 

b.  With  the  eyes  closed,  move  the  wrist  or  finger  over 
a  stationary  pencil-point.  In  this  case  the  point  also  seems 
to  be  in  motion  in  a  direction  contrary  to  that  of  the  hand. 


KIN^STHETIC  AND   STATIC  SENSES.  33 

c.  When  the  movement  may  be  interpreted  as  belonging 
to  either  of  two  members,  it  may  be  credited  to  the  more 
mobile  of  the  two,  or  may  be  shared  by  both.     Eest  the 
finger  lightly  on  the  forehead ;  then,  taking  pains  to  keep  its 
position  fixed,  move  the  head  from  side  to  side.     There  is  a 
strong  tendency  to  credit  the  motion  to  the  finger  and  arm. 
Hold  the  last  three  fingers  close  together,  and  move  the  first 
away  from  them  and  toward  them  again.     All  will  seem  to 
move,  but  the   last  three  in  an  opposite  direction  to  the 
first. 

d.  Ex.  4  above  is  an  experiment  on  the  location  of  move- 
ments as  well  as  of  touches.     If  the  cane  is  swung  so  as  to 
describe  the  surface  of  a  cone  we  are  conscious  of  the  path 
described  by  its  point,  as  well  as  that  of  the  hand  holding  it. 

Cf .  Ex.  39  where  the  motion  of  the  whole  fore-arm  and 
hand  is  credited  chiefly  to  the  latter. 
Vierordt;  on  c,  Goldsclieider,  A,  181  ff. 

42.  Interrupted  Extent  may  seem  smaller  to  a  moving  mem- 
ber than  uninterrupted.     In  a  piece  of  cardboard  make  three 
pin-holes  in  a  line  separated  by  spaces  of  an  inch  and  a  half. 
Fill  one  of  the  spaces  with  pin-holes  a  quarter  of  an  inch  apart. 
Turn  the  card  over,  close  the  eyes,  and  move  the  finger-tip 
across  the  little  eminences  made  by  the   pin-holes.     The 
illusion  seems  more  marked  when  the  finger  moves  over  the 
interrupted  half  of  the  line  first.     Examine  the  card  visu- 
ally, and  notice  that  the  visual  illusion  is  in  the  directly 
opposite  sense.     As  in  the  similar  touch  experiment  above 
(Ex.  8)  the  results  are  apparently  not  equally  clear  for  all 
observers. 

James,  II.,  250. 

SENSATIONS  OF  RESISTANCE. 

43.  Illusory  Resistance,     a.  Hold  a  heavy  weight  by  a 
string  so  that  it  hangs,  with  the  arm  extended,  a  few  inches 


34          LABORATORY  COURSE  IN  PSYCHOLOGY. 

above  the  floor,  or  better,  have  the  string  placed  in  the 
hand  by  an  assistant  so  that  the  length  of  the  string  may 
not  be  known  beforehand.  Lower  the  weight  rather  rapidly 
till  it  rests  on  the  floor  or  other  support.  As  it  strikes,  a 
sensation  of  arrest  will  be  perceived,  somewhat  as  though 
the  hand  were  suddenly  supported  by  a  light  rod.  The  illu- 
sion is  even  more  marked  when  the  string,  instead  of  being 
held  in  the  hand,  is  fastened  to  a  small  rod,  and  that  is 
held.  The  disturbing  noise  of  the  weight  may  be  obviated 
by  having  it  come  to  rest  on  a  cushion  or  in  a  box  of  sand. 
The  illusion  is  due  to  the  unexpected  strain  put  upon  the 
muscles  that  lower  the  arm  by  the  tension  of  those  that  have 
been  holding  the  weight.  This  feeling  of  arrest  is  prob- 
ably a  joint  sensation.  To  distinguish  this  sensation  from 
the  motion  sensations  of  the  joints,  Goldscheider  has  called 
it  a  "joint-pressure  sensation." 

b.  When  the  movement  of  the  rod  is  continued  downward 
beyond  the  point  at  which  the  sensation  of  arrest  is  felt,  a 
certain  difficulty  of  movement  may  still   be  observed,  as 
though  the  rod  were  moving  through  a  resisting  medium. 
This  sensation  Goldscheider  distinguishes  from  the  sensa- 
tion observed  in  #,  believing  it  to  be  the  true  sensation  of 
difficult  motion  (of  weight  and  heaviness  also)  and  credit- 
ing it  to  the  tendons. 

c.  Notice  something  similar  to  b  in  pouring  a  quantity 
of  mercury  rapidly  from  one  vessel  to  another. 

It  is  evident  that  such  illusions  as  these  speak  against 
the  existence  of  an  innervation  sense  in  the  common  accep- 
tation of  the  term. 

Goldscheider,  A,  164  ff.,  172  ff.,  D;  on  &,  A,  188;  Mach,  A,  70  ff. 

BILATERAL  ASYMMETRIES  OF  POSITION  AND  MOTION. 

44.  Apparently  Symmetrical  Motions  of  the  arms.  In  all 
the  tests  of  this  group,  the  subject  should  be  kept  in  ignor- 


AND   STATIC  SENSES.  35 

ance  of  the  nature  and  amount  of  his  errors  till  the  tests 
are  finished. 

a.  Hold,  an  ordinary  cork  between  the  thumb  and  first 
two  fingers  of  each  hand.     Close  the  eyes  and  bring  the  two 
corks  together  at  arm's  length  in  the  median  plane  before 
the  face,  having  an  assistant  note  the  approximate  amount 
and  direction  of  the  error.     The  corks  should  be  brought 
together  rather  gently,  so  as  not  to  betray  the  character  of 
the  error  to  the  operator,  but  the  motions  of  the  arms  by 
which  they  are  brought  up  nearly  to  contact  should  be  free 
and  sweeping.     The  error  will  probably  be  found  rather  con- 
stant in  direction  until  the  operator  learns   to  correct  it. 
Try  bringing  the  corks  together  above  the  head,  and  also  in 
asymmetrical  positions. 

b.  Let  the  subject  seat  himself  at  a  table  with  the  milli- 
meter scale  before  him.    Set  a  pin  in  the  middle  of  the  scale, 
and  bring  the  pin  into  the  median  plane  of  the  subject  and 
make  the  scale  parallel  to  his  frontal  plane.     Let  the  sub- 
ject place  his  forefingers  on  either  side  of  the  pin,  and,  with 
closed  eyes,  try  to  measure  off  equal  distances  by  moving 
both  simultaneously  outward  along   the    scale.     Note  the 
result  in  millimeters ;  for  this  it  may  be  convenient  to  mark 
the  middle  point  of  the  finger-nails  with  an  ink-line.     A 
constant  excess  in  the  motion  of  one  hand  or  the  other  will 
often  be  found.     It  is  important  that  the  subject  should  not 
open  his  eyes  till  his  fingers  are  removed  from  the  scale ; 
for  he  will  find  it  difficult  not  to  correct  his  error  if  he 
knows  its  nature.     The  finger-tips  should  rest  lightly  on 
the  scale,  and  the  motions  should  be  made  from  the  shoulder 
by  a  single  impulse ;  if  they  are  too  slow,  and  the  subject 
attends  to  his  sensations  of  position,  the  errors  will  be  small 
and  uncertain.    The  left  hand,  it  is  said,  generally  makes  the 
greater  excursion  in  right-handed  persons  not  mechanics. 

c.  Eepeat  the  tests,  having  the  motions  of  the  hands  made 


36          LABORATORY  COURSE  IN  PSYCHOLOGY. 

successively  instead  of  simultaneously.  The  constant  differ- 
ence between  the  hands  will  probably  not  appear. 

d.  Let  the  subject  start  with  his  right  and  left  hand  each 
20  cm.  toward  its  own  side  of  the  median  plane,  and  try 
to  measure  off  equal  distances  on  either  side  of  those 
points,  moving  both  hands  at  once  in  the  same  direction. 
Distances  inward  will  be  made  too  large,  distances  outward 
too  small.  In  all  these  experiments  with  closed  eyes  we 
seem  inclined  to  judge  distance  rather  from  the  intention 
of  equal  motion  and  the  continuance  of  motor  sensations 
for  equal  times,  than  from  the  actual  peripheral  sensations. 

The  judgments  of  symmetry  of  position  and  motion  rest 
upon  very  complex  combinations  of  the  dermal  and  kin- 
aesthetic  sensations,  already  made  the  subject  of  experiment 
above.  As  a  result  of  this  complexity  the  experiments  of 
this  group  will  be  found  to  give  rather  variable  results, 
from  one  subject  to  another,  and  in  the  same  subject  at 
different  times. 

Hall  and  Hartwell;  Loeb;  Delabarre;  Block. 

RECOGNITION  OF  THE  POSITION  OF  THE  BODY  AS  A  WHOLE. 
45.  Recognition  of  Direction.  In  this  experiment-  it  is 
especially  desirable  that  the  subject  should  know  as  little  as 
possible  of  the  purpose  of  the  experiment.  Cause  him  to 
stand  erect  with  his  back  against  a  wall.  Choose  a  point 
on  the  opposite  wall  about  the  height  of  his  shoulders.  Let 
him  look  at  it,  and  then  require  him,  having  closed  his  eyes, 
to  point  to  it  as  exactly  as  possible  with  a  light  rod  held 
symmetrically  in  both  hands.  Cause  him  also  to  hold  the 
rod  vertically  and  horizontally  in  the  median  plane ;  also 
horizontally  parallel  to  the  frontal  plane.  All  these  he  will 
probably  be  able  to  do  with  much  accuracy ;  or  if,  as  some- 
times happens,  he  shows  a  "personal  equation,"  his  error 
will  be  constant. 


KIN^STHETIC  AND   STATIC  SENSES.  37 

a.  Cause  the  subject  to  repeat  the  experiment,  this  time 
turning  his  head  as  far  as  possible  to  the  left  after  closing 
his  eyes,  taking  pains  to  keep  his  shoulders  square.     Eepeat, 
causing  the  subject  to  turn  to  the  right.     In  both  cases  an 
error  will  be  observed,  the  subject  pointing  too  far  in  a 
direction  opposite  to  that  of  the  turning  of  the  head.     The 
subjeckwill  be  able  to  hold  the  rod  vertically,  or  horizontally, 
without  error.     Cause  the  subject  to  hold  the  rod  in  what 
he  thinks  is  a  horizontal  position,  in  the  median  plane  when 
his  head  is  thrown  well  back;  when  bowed  well  forward. 
Illusions  like  those  observed  above,  affecting  directions  in 
the  plane  of  movement  of  the  head,  will  result.     Cause  the 
subject  to  hold  the  rod  in  what  he  thinks  is  a  horizontal 
position,  parallel  to  the  frontal   plane,  when  his  head  is 
bowed  to  the  right ;  when  bowed  to  the  left.     Illusions  sim- 
ilar to  those  in  the  previous  experiments  will  appear.     In  all 
these  cases  judgment  of  one  cardinal  direction  in  space  alone 
is  affected  ;  the  other  two  show  little  or  no  errors. 

b.  Eepeat  the  first  part  of  experiment  a;  but  instead  of 
having  the  subject  point  to  the  designated  object,  have  him 
walk  toward  it,  keeping  his  shoulders  square,  his  eyes  shut, 
and  his  head  turned  to  one  side.     He  will  walk  more  and 
more  too  far  toward  the  side  away  from  which  his  head  is 
turned. 

c.  The  illusion  is  due,  at  least  in  the  case  of  turning  the 
head  about  a  vertical  axis,  to  the  position  of  the  eyes ;  the 
eyes  turn  farther  than  the  head  in  the  direction  in  which 
it  is  turned,  as  may  easily  be  observed  upon  any  other  per- 
son.    From  the  eyes  we  judge  the  position  of  the  head,  and 
thus  over  judging  it,  point  too  far  in  a  contrary  direction  in 
trying  to  point  to  the  required  object  (Delage).     The  illu- 
sions can  be  produced  by  motion  of  the  eyes  alone.     Holding 
the  head  erect,  and  taking  pains  not  to  move  it  when  moving 
the  eyes,  turn  the  closed  eyes  as  far  as  possible_fcQjhe  right 


38          LABORATORY  COURSE  IN  PSYCHOLOGY. 

or  left,  and  then  try  to  point  to  some  determined  object.  An 
error  like  that  in  a  will  be  observed.  Turning  of  the  eyes 
upward  or  downward  has  a  doubtful  result.  Instead  of 
closing  the  eyes,  they  may  be  kept  open  if  an  opaque 
screen  is  held  close  before  the  face.  Eepeat  a,  voluntarily 
turning  the  eyes  as  far  as  possible  in  the  direction  opposite 
to  that  of  the  turning  of  the  head.  The  original  error  will 
probably  disappear,  or  be  found  to  have  changed  its  sign. 

For  this  illusion  another  eye  explanation  is  suggested  by 
Breuer,  namely,  that  in  such  extreme  turnings  of  the  eyes, 
their  actual  position  does  not  correspond  with  the  intended 
position,  but  conies  short  of  it.  We  infer  the  direction, 
however,  from  the  intended  position,  and  thus  fall  into  the 
error  in  pointing.  For  the  illusion  in  other  positions  of  the 
head  and  even  for  this,  his  own  preferred  explanation  is 
again  different,  and  is  partly  based  on  the  following  experi- 
ment. 

d.  Close  the  eyes,  and  touch  the  tip  of  the  nose  or  the 
forehead  with  a  pin  or  a  pencil  while  the  head  is  in  the 
usual  position,  and  after  a  little  try  to  touch  the  same  spot 
again.  The  error,  if  any,  will  be  very  small.  Eepeat  the 
touch  in  the  normal  position,  and  then  turn  the  head  to  the 
right  or  left  or  incline  it  toward  the  shoulder  or  forward  or 
backward.  After  holding  it  in  the  chosen  position  for  half 
a  minute,  attempt  to  touch  the  spot  again.  Gross  errors 
will  result  till  corrected  by  practice.  The  error  is  one  of 
underestimation,  and  should  by  itself  alone  produce  a  result 
directly  the  reverse  of  that  found  by  Delage.  Breuer,  how- 
ever, introduces  another  factor.  His  explanation  for  the 
inclined  positions  of  the  head  is  somewhat  as  follows  :  by 
means  of  the  otolith-apparatus  of  the  ear,  we  get  a  true 
perception  of  the  amount  of  inclination  of  the  head,  at  the 
same  time  that  we  get  the  erroneous  perception  just  men- 
tioned. The  only  way  in  which  we  can  harmonize  the 


KIN^STHETIC  AND   STATIC  SENSES.  89 

Conflicting  perceptions  is  by  altering  our  judgment  of  the 
vertical,  and  with  that,  of  course,  of  the  horizontal.  For  the 
movements  of  rotation  about  a  vertical  axis  the  semi-circular 
canals  (See  Exs.  47-49)  would  furnish  the  knowledge  of 
the  true  amount  of  turning,  and  from  a  similar  combination 
of  the  true  and  false  the  illusions  in  that  case  would  result. 

This  group  of  experiments,  except  perhaps  the  last,  when 
tried  under  the  ordinary  conditions  of  the  practice  labora- 
tory, seems  liable  to  considerable  individual  variation ;  but 
sufficient  care,  especially  as  to  the  position  of  the  eyes  in 
turning  to  the  right  and  left,  should  lead  to  a  tolerable 
degree  of  success. 

Aubert  (Delage),  17  ff.;  Loeb,  B,  20  f.,  31  f.;  Breuer,  270  ff. 

46.  Vertical  and  Horizontal  Positions  of  the  Body.  Secure 
the  subject  properly  upon  the  tilt-board,  and  have  him  close 
his  eyes.  Start  with 
the  board  vertical  (head 
up).  Require  the  sub- 
ject to  describe  his  po- 
sition. He  will  prob- 
ably announce  that  he 
is  then  leaning  forward 
slightly.  As  a  matter 
of  fact  he  is,  if  his  heels 
are  against  the  board. 
Turn  him  slowly  back- 
ward, and  require  him 
to  say  when  he  seems  to 
himself  vertical  (head 
up),  when  he  seems 
tilted  backward  at  an  angle  of  45°  from  the  vertical,  when  at 
an  angle  of  60°,  when  at  90°,  when  at  180°.  Two  classes  of 
illusions  will  be  found :  angles  of  less  than  40°  will  prob- 
ably seem  too  small ;  those  from  40°  to  60°  will  be  rightly 


40          LABORATORY  COURSE  IN  PSYCHOLOGY. 

i 

judged  ;  those  beyond  60°  will  seem  too  large.  The  subject 
will  say  that  he  is  vertical,  head  downward,  when  he  is  yet 
30-60°  from  it.  The  subject  may  be  allowed  a  pillow  if  he 
desires  it. 

The  illusions  depend  in  large  measure  on  the  distribution 
of  pressure  on  the  soles  and  other  surfaces  of  the  body  and 
the  direction  of  pressure  of  the  movable  viscera  and  the 
blood. 

Aubert  (Delage),  40  ff. ;  Breuer,  270  f. 

SENSATIONS  OF  ROTATION. 

47.  Perception  of  Uniform  Eotations.     Let  the  subject 
be  seated  upon  the  rotation  table  with  closed  eyes,  blind- 
folded if  necessary.     Turn  the  table  slowly  and  evenly  in 
one  direction  or  the  other.     The  subject  will  immediately 
recognize  the  direction  and  approximately  the  amount  of 
rotation  when  the  rate  is  as  m  slow  as  2°  per  second,  or  even 
slower.      After  continued   rotation  at  a  regular   rate   the 
sensation  becomes  much  less  exact  or  entirely  fails.     This 
fact  has  been  generalized  by  Mach  in  the  law  that  only 
change  of  rate,  not  continuous  rotation,  is  perceived.     After 
some  pauses  and  short  movements  in  one  direction  and  the 
other,  the  subject  may  become  quite  lost,  and  give  a  totally 
wrong  judgment  of  the  direction  of  motion,  if  it  is  slow. 

48.  Illusion  of  Backward  Eotation.     Let  the  subject  be 
seated  as  before.     Rotate  him  a  little  more  rapidly  for  half 
a  turn,  and  then  stop  him  suddenly.     A  distinct  sensation 
of  rotation  in  the  opposite  direction  will  result.     Repeat, 
and  when  the  illusory  rotation  begins,  open  the  eyes.     It 
immediately  ceases.     Close  the  eyes  again,  and,  if  strong,  it 
again  returns. 

49.  Location  of  the  Organs  for  the  Perception  of  Rotation. 
a.  Repeat  the  first  part  of  Ex.  48,  letting  the  subject 


KIN  ESTHETIC  AND   STATIC  SENSES.  41 

give  the  word  for  stopping.  At  the  same  instant  let  him 
incline  his  head  suddenly  backward  or  forward,  or  lay  it 
upon  one  shoulder  or  the  other.  •  The  axis  of  rotation  of 
the  body  will  appear  to  change  in  a  direction  opposite  to 
that  of  the  inclination  of  the  head ;  i.e.,  if  the  head  is  in- 
clined to  the  right,  the  axis  seems  to  incline  to  the  left. 
The  feeling  is  as  if  the  body  were  rotating  in  the  surface 
of  a  cone  in  a  direction  contrary  to  that  of  the  first  rotation. 
The  head  dictates  the  apparent  axis  of  rotation.  The  same 
illusion  occurs  if  the  head  is  inclined  during  the  actual 
rotation  and  straightened  at  the  word  for  stopping.  Turning 
the  head  to  the  right  or  left  introduces  no  such  illusion, 
because  it  does  not  change  the  axis  of  rotation  of  the  head. 
The  illusion  comes  out  with  very  disagreeable  strength 
when  the  rotation  is  rapid,  and  the  subject  changes  the 
position  of  his  head  during  the  rotation. 

b.  Let  the  subject  lie  upon  his  side,  and  rotate  him  rather 
rapidly  till  the  sensation  of  rotation  becomes  faint  or  disap- 
pears. Then  let  him  turn  suddenly  upon  his  back  or  upon 
his  other  side.  Turning  upon  his  back  starts  rotation  about 
a  new  axis,  and  it  is  felt  in  its  true  sense,  while  the  rotation 
about  the  previous  axis  is  felt  as  an  illusion  in  its  reverse 
sense.  The  resulting  perception  combines  both.  Turning 
completely  over  reverses  the  direction  of  motion  completely, 
and  the  combined  sensation  and  illusion  produce  a  corre- 
spondingly powerful  effect. 

The  change  of  the  apparent  axis  of  rotation  with  the 
change  of  position  of  the  head  points  to  the  location  in  the 
head  of  the  organ  for  such  sensations.  For  the  experiments 
by  which  the  semicircular  canals  are  indicated  as  this  organ, 
and  the  arguments  pro  and  con,  see  the  literature  cited  by 
Aubertj  Ayres,  and  others. 

On  the  last  three  experiments,  see:  Aubert  (Delage),  49  ff. ; 
Brown;  Mach;  Wundt,  3te  Aufl.,  L,  211  f.;.IL,  24,139. 


42          LABORATORY   COURSE  IN  PSYCHOLOGY. 

..  50.  Another  Illusion  of  Eotation  (Purkinje's  dizziness) 
is  due  to  involuntary  motions  of  the  eyes.  Let  the  subject 
whirl  rapidly  on  his  heels  with  his  eyes  open  till  he  begins 
to  be  dizzy.  At  first  objects  about  him  seem  at  rest,  then 
to  be  turning  in  the  opposite  direction.  Let  him  now  stop 
and  look  at  an  even  surfaced  wall  while  the  experimenter 
carefully  observes  his  eyes,  picking  out  some  clearly  marked 
fleck  or  spot  as  a  point  of  observation.  To  the  subject  the 
surrounding  objects  will  seem  to  continue  to  move  in  the 
same  direction  as  before ;  i.e.,  in  a  direction  contrary  to  his 
previous  rotation;  the  experimenter  will  see  the  subject's 
eyes  executing  slow  motions  in  one  direction  (in  the  direc- 
tion of  the  original  motion  of  the  subject)  alternating  with 
rapid  motions  in  the  other.  The  subject  himself  may  be 
able  to  perceive  a  corresponding  irregularity  of  motion  in 
the  spots  upon  the  wall  at  which  he  looks.  He  can  easily 
observe  the  motions  of  his  own  eyes  if  he  looks  fixedly  for 
twenty  or  thirty  seconds  at  a  flame  or  a  strip  of  white  paper 
in  a  bright  light  before  beginning  his  rotation ;  the  after- 
image (see  Chapter  Y.)  thus  produced  remains  fixed  on  the 
retina,  and  its  apparent  movements  betray  the  motions  of 
the  eye.  If  the  eyes  are  closed  after  the  rotation,  the  image 
will  seem  to  move  in  one  direction,  and  rather  slowly.  The 
illusion  rests  upon  the  subject's  unconsciousness  of  the 
slow  motions  of  his  eyes.  It  is  probable  that  these  eye 
motions  and  the  sensations  of  attempted  restoration  of  equi- 
librium in  other  parts  of  the  body  are  reflexly  caused  by  the 
disturbance  in  the  semicircular  canals. 

It  should  be  noticed  that  this  illusion  is  the  exact 
reverse  of  that  found  with  closed  eyes  in  Ex.  48.  There  the 
subject  feels  a  rotation  of  his  own  body  contrary  to  that  it 
previously  received.  If  he  was  turned  at  first  in  the  direction 
of  the  hands  of  a  watch,  on  being  stopped  he  would  seem  to 
be  turning  in  a  direction  contrary  to  the  hands.  If  these 


KIN  AESTHETIC  AND    STATIC   SENSES.  43 

motions  were  transferred  to  objects  about  him,  they  would, 
during  the  rotation,  seem  to  move  contrary  to  the  hands, 
and  after  stopping,  in  the  direction  of  the  hands.  In  the 
Purkiiije  experiment  the  motion  of  objects  is  not  thus  re- 
versed. 

Those  who  try  these  rotational  experiments  should  do  so 
with  caution,  for  the  unpleasant  effects  of  them  sometimes 
last  several  hours. 

Aubert  (Delage),  52,  100  ff. ;  Mach.  Aubert  reprints  Purkinje's 
paper  on  dizziness  as  an  appendix  to  the  translation  of  Delage. 

SENSATIONS  OF  PROGRESSIVE  MOTION. 

51.  Progressive  motions,  so  far  as  they  do  not  involve 
rotation,  probably  give  us  combinations  of  sensations  from 
several  different  sources.  The  principle  holds  for  progres- 
sive motions  as  for  rotations,  that  we  perceive  changes  of  rate 
of  motion,  and  not  uniform  motion  ;  as  long  as  the  motion 
remains  uniform  we  can  by  an  effort  of  imagination  conceive 
ourselves  to  be  moving  in  either  direction  or  to  be  standing 
still,  except  for  what  jarring  there  may  be.  The  apparatus 
for  the  study  of  these  phenomena  will  be  found  in  railroad 
trains  and  elevators.  See  also  Mach  for  special  laboratory 
apparatus. 

Aubert  (Delage),  75  ff. ;  Mach;  Brown;  Breuer,  283. 


BIBLIOGKAPHY. 

AUBERT  :  Physiologische  Studien  iiber  die  Orientierung,  Tubingen, 
1888,  122.  This  is  a  translation  of  the  paper  of  Delage 
below,  with  full  notes  and  appendix  containing  Purkinje's  Bul- 
letin von  1825,  Ueber  den  Schwindel,  See  criticism  by  Breuer, 
270  ff. 


44          LABORATORY  COURSE  IN  PSYCHOLOGY. 

AYRES  :  A.  A  Contribution  to  the  Morphology  of  the  Vertebrate 
Ear,  with  a  Keconsideration  of  its  Functions,  Journal  of 
Morphology,  VI.,  Nos.  1  and  2,  May,  1892.  Ayres  gives  a 
bibliography  of  nearly  three  hundred  titles,  many  upon  the 
psycho-physiology  of  the  semicircular  canals,  but  not  a  complete 
list. 
B.  The  Ear  of  Man  :  Its  Past,  Present,  and  Future.  Wood's 

Holl  Biological  Lectures,  1890.     An  Abstract  of  A. 
BASTIAN  :    "The  Muscular  Sense;  "  Its  Nature  and  Cortical  Locali- 
sation, .Brain,  X.,  1887-88,  1-89.     Discussion  on  the  paper  by 
Ferrier,  Sully,  and  others,  89-137. 
BEAUNIS:   Les  Sensations  internes,  Paris,  1889. 
BLOCK  :  Experiences  sur  les  sensations  musculaires,  Revue  Scien* 
tifique,  XLV.,  No.  10,  1890,  294-301. 

BBEUEB:  Ueber  die  Function  der  Otolithenapparate,  Pfluger's 
Archiv,  XL VIII.,  1890-91,  195-304. 

BROWN:   A.   On  the  Sense  of  Kotation  and  the  Anatomy  and  Physi- 
ology of  the  Semicircular  Canals  of  the  Internal  Ear,  Journal 
of  Anatomy  and  Physiology,  VIII.,  1874,  327.     Reprinted  by 
Mach,  A,  100. 
B.    On  Sensations  of  Motion,  Nature,  XL.,  1889,  449. 

CHARPENTIER  :  Analyse  experimental  de  quelques  elements  de  la 
sensation  de  poids,  Archives  de  Physiologic,  Ser.  5,  III.,  1891, 
122-135. 

DELABARRE:  Ueber  Bewegungsempfindungen,  Inaug.  Diss.,  Frei- 
burg, 1891,  111.  The  author  has  also  published  a  portion  of 
the  same  matter  in  Mind,  Ser.  2,  1892,  379-396. 

DELAGE:  fitudes  experimentales  sur  les  illusions  statiques  et  dy- 
namiques  de  direction  pour  servir  a  determiner  les  fonctions 
des  canaux  demicirculaires  de  1'oreille  interne,  Archives  de 
Zool.  Exper.,  No.  4,  1886,  535-624  (with  index).  Translated 
by  Aubert  above.  See  also  abstract  of  this  paper  in  Comptes 
rendus,  CIIL,  1886,  749. 

EWALD:  Physiologische  Untersuchungen  iiber  das  Endorgan  des 
Nervus  octavus,  Wiesbaden,  1892.  Gives  a  very  extended 
bibliography. 

FERBIER  :  Functions  of  the  Brain,  London,  1886. 


KIN^ESTHETIC  AND   STATIC  SENSES.  45 

FULLERTON  AND  CATTELLi  On  the  Perception  of  Small  Differ- 
ences, Publications  of  the  University  of  Pennsylvania,  Philo- 
sophical Series,  No.  2,  Philadelphia,  1892. 

FUNKE:  Der  Tastsinn  und  die  Gemeingefuhle,  Hermann's  Handbuch 
der  Physiologic,  III.,  pt.  2,  289-414. 

GOLDSCHEIDER  i  A.  Untersuchungen  iiber  den  Muskelsinn,  Du  Bois- 
Reymond's  Archiv,  1889,  369  ff.  and  540,  also  Supplement- 
Band,  1889,  141  ff. 
B.   Ueber  den  Muskelsinn  und  die  Theorie  der  Ataxie,  Zeitschrift 

fur  klin.  Med.,  XV.,  1888-89. 
(7.   Ueber  die  Grenzen  der  Wahrnehmung  passiver  Bewegungen, 

Centralblattfur  Physiologic,  I.,  1887,  223-225. 
D.  Ueber  paradoxe  Widerstandsempfindung,  Ibid,  III.,  1889,  90-91. 

HALL  AND  HARTWELL:  Bilateral  Asymmetry  of  Function,  Mind, 
IX.,  1884,  93-109. 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 

KREIDL  :  Beitrage  zur  Physiologic  des  Ohrlabyrinthes  auf  Grand  von 
Versuchen  an  Taubstuminen,  Pfluger's  Archiv,  LI.,  1891-92, 
119-150. 

LOEB:  A.  Untersuchungen  iiber  den  Fiihlraum  der  Hand;  Erste 
Mittheilung,  Gleiche  Fiihlstrecken,  Pfluger's  Archiv,  XLL, 
1887,  107-127. 

B.  Untersuchungen  iiber  die  Orientirung  im  Fiihlraum  der  Hand 
und  im  Blickraum,  Ibid.  XL VI.,  1890,  1-46. 

MACH:  A.  Grundlinien  der  Lehre  von  den  Bewegungsempfindungen, 

Leipzig,  1875,  128.     Gives  bibliography  of  thirty-one  titles. 
B.  Analyse  der  Empfindungen,  Jena,  1886,  69  ff. 

MULLER  UND  SCHUMANN:  Ueber  die  psychologischen  Grundlagen 
der  Vergleichung  gehobener  Gewichte,  Pfliiger's  Archiv,  XLY., 
1889,  37-112. 

MUNSTERBERG:  Die  Willenshandlung,  Freiburg,  1888. 

SCHAEFER:  Die  Erklarung  der  Bewegungsempfindungen  durch  den 
Muskelsinn,  Inaug.  Diss.,  Jena,  1889;  also  an  article  of  similar 
title,  Pfluger's  Archiv,  LXL,  1887,  566-640. 

STERNBERG  :  Zur  Lehre  von  den  Vorstellungen  iiber  die  Lage  un- 
serer  Glieder,  Pfluger's  Archiv,  XXXYII.,  1885,  1.  Gives 
bibliography  of  fifty-two  titles. 


46          LABOEATOEY  COURSE  IN  PSYCHOLOGY. 

VIERORDT:  Die  Bewegungsempfindung,  Zeitschrift  filr  Biologie, 
XII. ,  1876,  226-240.  See  also  Vierordt's  Physiologic,  5te  Aufl., 
329  ff. 

WALLER:  The  Sense  of  Effort,  Brain,  XIY.,  1891,  179-249, 
433-436,  especially  229  ff.  This  study  is  accompanied  by  a 
bibliography  of  fifty  titles.  Nearly  the  same  portion  of  the 
paper  indicated  here  as  of  special  importance  will  be  found 
under  the  following  title :  Experiments  on  Weight-discrimina- 
tion, Proceedings  of  the  Physiological  Society,  Session  of  Jan. 
30,  1892,  Journal  of  Physiology,  XIII. ,  May,  1892. 

WEBER:  Work  cited  in  bibliography  of  Chap.  I. 

WLASSAK:  Die  statischen  Functionen  des  Ohrlabyrinthes  und  ihre 
Beziehungen  zu  den  Eaumempfindungen,  Vierteljahr.  fur  iviss. 
Philosophic,  XVL,  1892,  385-403,  XVII.,  1893,  15-29. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3te  Aufl.,  I.,  397  ff., 
II.,  21  ff.;  4te  Aufl.,  I.,  419  ff. 


SENSATIONS   OF   TASTE  AND  SMELL.  47 


CHAPTER   III. 
Sensations  of  Taste  and  Smell. 

THESE  sensations  are  of  secondary  importance  in  psy- 
chology, and  have  received  a  correspondingly  small  share 
of  investigation.  In  subjective  quality  they  seem  to  stand 
midway  between  the  general  senses  mentioned  at  the  end 
of  Chapter  I.  and  the  higher  senses  of  Hearing  and  Vision. 

SENSATIONS  OF  TASTE. 

52.  Tastes   and   Smells.      Much   of  what  is   commonly 
called  taste  is  really  a  combination  of  taste  with  smell  and 
with  touch  in  its  various  forms.     With  the  nostrils  held,  try 
to  distinguish  by  taste  alone  between  small  quantities  of 
water  and  a  weak  solution  of  essence  of  clove  in  water. 
A  discrimination  that  is  easily  possible  with  the  nostrils 
open  is   difficult   or  impossible  with   the   nostrils   closed. 
The  solution  should  not  be  swallowed,  for  then  the  olfac- 
tory region  may  be  reached  from  the  back  of  the  nose. 

53.  Distribution  of  the  Organs  of  Taste,     a.  Using  the 
weaker  taste  solutions,  and  operating  upon  yourself  with  a 
mirror  or  on  another  person,  find  out  as  nearly  as  you  can 
in  what  part  of  the  tongue  the  strongest  sensations  are  pro- 
duced by  each.     Test  the  tip,  the  sides,  the  back,  and  the 
middle,  putting  the  solutions  on  with  a  cameFs-hair  brush, 
and  rinsing  the  mouth  as  often  as  necessary.     Try  also  the 
hard  and  soft  palates. 

b.  Dry  the  tongue  with  a  handkerchief,  and  test  the  in- 
dividual fungiform  papillae  with  the  stronger  solutions, 


48          LABORATORY  COURSE  IN  PSYCHOLOGY. 

applying  them  with  fine  camel's-hair  pencils.  It  will  be 
found  possible  to  get  taste  sensations  from  the  single 
papillae,  though  perhaps  not  all  four  from  each.  Einse  the 
mouth  as  needed.  Test  the  surface  of  the  tongue  between 
the  papillae  and  observe  that  no  taste  sensations  follow. 

a.  Rittmeyer;  b.  Oehrwall. 

54.  Minimal  Tastes,    a.  Find  what  is  the  greatest  dilution 
of  the  weaker  solutions  in  which  the  characteristic  tastes  can 
still  be  recognized.    The  same  quantity,  e.g.,  half  a  teaspoon- 
ful,  should  be  taken  into  the  mouth  at  each  trial,  and  may 
be  swallowed  with  advantage.     Einse  the  mouth  thoroughly 
as  required.     The  following   are   the  average  proportions 
found  by  Bailey  and  Nichols  for  male  observers :  Quinine, 
1 : 390  000 ;  Sugar,  1 : 199 ;  Salt,  1  : 2240 ;  for  Sulphuric  Acid, 
which  they  used  instead  of  Tartaric,  the   proportion  was 
1  : 2080. 

b.  The  intensity  of  the  sensation  and  the  greatest  dilution 
still  tastable  depend  on  the  number  of  taste  organs  stimu- 
lated.    Take  a  portion  of  one  of  the  solutions  of  just  tast- 
able strength,  found  in  a,  add  an  equal  quantity  of  water, 
and  take  a  large  mouthful  of  the  mixture.     The  character- 
istic taste  will  still  be  perceived,  perhaps   more  strongly 
than  before. 

a.  Bailey  and  Nichols,  A\  Lombroso  und  Ottolenghi;  Camerer,  A. 
b.  Camerer,  B. 

55.  Discriminative   Sensibility  for  Taste.     For  a  rough 
determination,  test  with  solutions  of  sugar,  taking  first  a 
small  quantity  of  the  standard  20%  solution,  then  an  equal 
quantity  (the  equality  is  important)  of  one  of  the  weaker 
solutions,  or  first  one  of  the  weaker  and  then  the  standard, 
until  a  solution  is  found  that  is  just  recognizably  different 
from  the  standard.     Make  this  determination  several  times. 
The  excess  of  sugar  in  the  standard  solution  over  the 


SENSATIONS   OF   TASTE  AND  SMELL.  49 

amount  in  the  solution  just  observably  weaker,  set  in  a  ratio 
to  the  total  percentage  of  sugar  in  the  standard,  measures 
the  sensibility.  Some  experimenters  may  be  able  to  dis- 
tinguish the  18%  from  the  20%  solution;  their  sensibility 
would  then  be  expressed  by  the  ratio  2  : 20. 
Keppler. 

56.  Electrical  Stimulation,     a.  Using  a  constant  current, 
from  a  single  Grenet  cell,  for  example,  and  two  small  zinc 
electrodes,  one  applied  to  the  inner  surface  of  the  under  lip 
and  the  other  to  the  tongue,  notice  the  sour  taste  at  the 
positive  pole  and  the  alkaline  at  the  negative. 

Von  Yintschgau,  181  ff. ;  Oehrwall ;  Hermann. 

SENSATIONS  OF  SMELL. 

57.  Minimal   Odors.      The  keenness   of   smell  may  be 
tested  with  dilute  solutions  of  odorous  substances  or  with 
the   olfactometer. 

a.  Test  with  solutions.  Pour  small  quantities  of  the 
solutions  of  oil  of  cloves  into  little  wide-mouthed  bottles, 
filling  each  to  about  the  same  height.  Mark  all  in  an  in- 
conspicuous manner.  Set  the  bottles  a  foot  apart  on  a 
table  in  a  place  where  there  is  moderate  circulation  of 
air,  in  the  order  of  the  strength  of  their  solutions,  be- 
ginning with  the  water  and  following  with  the  weakest  so- 
lution and  so  on.  Require  the  subject  to  smell  of  the 
bottles  in  succession  without  lifting  them  from  the  table, 
beginning  with  the  water,  and  to  indicate  that  in  which  he 
first  recognizes  a  characteristic  odor.  If  the  solutions  stand 
for  any  length  of  time  where  they  are  subject  to  evaporation, 
it  will  be  safer  to  prepare  fresh  ones  before  undertaking  a 
new  test.  Other  precautions  will  suggest  themselves,  such 
as  the  use  of  similar  bottles,  and  care  in  filling  them  that 
none  of  the  solution  is  left  clinging  near  the  mouth. 


50           LABORATORY  COURSE  IN  PSYCHOLOGY. 

b.  Test  with  the  olfactometer.  Test  the  sides  of  the  nose 
separately.  Push  the  odor-tube  on  till  its  end  is  flush  with 
that  of  the  glass  tube,  insert  the  bent  end  of  the  latter  into 
the  nostril,  and  gradually  lengthen  the  exposed  surface 
of  the  odor-tube  till  its  odor  is  just  discernible.  Note  in 
millimeters  the  length  exposed. 

a.  Bailey  and  Nichols,  B\  Lombroso  und  Ottolenghi;  Savelieff; 
b.  Zwaardemaker,  A  and  C. 

58.  Discriminative    Sensibility   for    Odors.     Using   the 
double   olfactometer  with   both   odor-tubes  drawn  out  far 
enough  to  give  an  unmistakable  odor,  but  not  too  strong  a 
one,  say  both  drawn  out  5  cm.,  find  how  far  one  or  the  other 
must  be  drawn  out  (or  pushed  back)  to  make  the  odor  which 
it  gives  just  observably  stronger  (or  weaker)  than  that  of 
the  other.     The  test  should  be  made  with  the  sides  of  the 
nose  separately  (there  is  frequently  a  difference  in  sensi- 
tiveness between  the  two  sides,  due  to  mechanical  obstruc- 
tion or  other  cause),  unless  for  some  reason  a  bilateral  form 
of  experiment  is  desirable.     Try  a  number  of  times,  in  half 
the  tests  smelling  the  weaker  before  the  stronger,  and  in 
half  the  stronger  before  the  weaker,  but  be  careful  to  avoid 
fatigue. 

59.  Fatigue  of  Smell,     a.  Hold  a  piece  of  camphor  gum 
to   the   nose,   and   smell  of  it  continuously,  breathing   in 
through  the  nose  and  out  through  the  mouth,  for  five  or  ten 
minutes.     A  very  marked  decrease  in  the  intensity  of  the 
sensation  will  be  observed,  reaching  perhaps  even  to  com- 
plete loss  of  the  odor. 

b.  It  is  important,  however,  to  observe  that  fatigue  for  one 
substance  does  not  cause  obtuseness  for  all  other  substances, 
though  it  does  for  some.     Smell  of  some  essence  of  cloves 
and  of  some  yellow  wax,  then  fatigue  for  camphor  as  in  a, 
and  smell  of  the  essence  of  cloves  and  of  the  wax  again. 


SENSATIONS   OF   TASTE  AND  SMELL.  51 

The  odor  of  the  wax  will  probably  be  fainter,  that  of  the 
essence  of  cloves  unaffected. 
Aronsohn. 

60.  Combination  of  Odors.  Experiment  with  the  olfac- 
tometer  on  one  side  of  the  nose  as  follows.  Hold  against 
the  end  of  the  rubber  odor-tube  another  odor-tube  of  wax 
(partly  covered  on  the  inside  by  a  glass  tube  of  the  same 
size  as  that  used  in  the  olfactometer),  in  such  a  way  that 
the  air  must  pass  through  both  to  reach  the  nose.  Then 
gradually  increase  the  length  of  the  rubber  tube  exposed 
till  the  odor  of  the  wax  is  no  longer  perceived.  If  the 
experiment  is  carefully  performed,  a  point  may  be  found 
where  the  odors  nearly  balance.  If  the  rubber  is  length- 
ened beyond  this  point,  its  odor  overpowers  that  of  the 
wax ;  if  it  is  shortened,  it  is  overpowered  by  that  of  the 
wax.  A  mixture  of  the  odors  in  which  both  can  be  detected 
is  difficult  to  find.  Care  should  of  course  be  taken  to  avoid 
fatigue. 

A  similar  balance  of  odors  was  found  by  Zwaardemaker 
when  the  double  olfactometer  was  used  and  the  two  sides  of 
the  nose  received  separate  stimuli. 

Zwaardemaker,  B. 


BIBLIOGRAPHY. 

ARONSOHN:  Experimentelle  Untersuchungen  zur  Physiologic  des 

Geruchs,  Du  Bois-Reymond' s  Archiv,  1886,  321-357. 
BAILEY  AND  NICHOLS:  A.   The  Delicacy  of  the  Sense  of  Taste, 

Nature,  XXXVII.,  1887-88,  557;  also  Science,  1888,  145. 
B.   The  Sense  of  Smell,  Nature,  XXX V.,  1886-87,  74. 
CAMERER:  A.  Die  Grenzen  der  Schmeckharkeit  von  Chlornatrium 

in  wasseriger  Losung,  P finger's  Archiv,  II.,  1869,  322. 
B.   Die  Methode  der  richtigen  und  falschen  Fiille  angewendet  auf 
den  Geschmackssinn,  Zeitschrift  fur  Biologie,  XXL,  570. 


52          LABORATORY  COURSE  IN  PSYCHOLOGY. 

COBIN:  Action  des  acides  sur  le  gout,  Archives  de  Biologie,  VIII. , 
1888,  fasc.  1,  121-138. 

GOLDSCHEIDER  UNO  SCHMIDT  :  Bemerkungen  iiber  den  Geschmack- 
sinn,  Centralblattfur  Physiologic,  IV.,  1890,  10-12. 

HAYCRAFT:  The  Nature  of  the  Objective  Cause  of  Sensation;  Taste, 
Brain,  X.,  1887,  145-163;  Smell,  Ibid.,  XL,  1888-89,  166-178. 

HERMANN:  Beitrage  zur  Kenntniss  des  elektrischen  Geschmacks, 
nach  Versuchen  von  Laserstein,  Pfliiger's  Archiv,  XLIX.,  1891, 
519-538. 

HOWELL  AND  KASTLE  i  Note  on  the  Specific  Energy  of  the  Nerves 
of  Taste,  Studies  from  the  Biological  Laboratory,  Johns  Hopkins 
University,  IV.,  No.  1. 

KEPPLER:  Das  Unatrscheidungsvermogen  des  Geschmacksinnes  fiir 
Concentrationsdifferenzen  der  schmeckbaren  Korper,  Pfluger's 
Archiv,  II.,  1869,  449. 

LOMBROSO  UND  OxTOLENGHi :  Die  Sinne  der  Verbrecher,  Zeitschrift 
fiir  Psychologic,  II.,  1891,  342-348. 

OEHRWALL:  Untersuchungen  liber  den  Geschmackssinn,  Scandinav. 
Archiv  f.  PhysioL,  II. ,  1890,  1-69;  see  also  abstract  by  the 
author  in  the  Zeitschrift  f.  Psych.,  I.,  1890,  141. 

PASSY:  Sur  les  minimums  perceptibles  de  quelques  odeurs,  Comptes 
rendus,  CXIV.,  1892,  306. 

KAMSEY:  On  Smell,  Nature,  XXVI.,  1882,  187. 

KITTMEYER:  Geschmackspriifungen,  Inaug.  Diss.,  Gottingen,  1885. 

SAVELIEFF  :  Untersuchung  des  Geruchsinnes  zu  klinischen  Zwecken, 
Neurologisches  Centralblatt,  XII.,  1893,  340-345. 

SHORE:  A  Contribution  to  our  Knowledge  of  Taste  Sensations, 
Journal  of  Physiology,  1892,  191-217. 

VON  VINTSCHGAU:  Physiologic  des  Geschmackssinns  und  des  Ge- 
ruchssinns,  Hermann's  Handbuch  der  Physiologic,  III.,  pt.  2, 
143-286.  For  a  general  account  of  the  physiology  and  psychol- 
ogy of  taste  and  smell  to  1880,  and  references  to  the  earlier 
literature,  see  this  work. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3te  Aufl.,  I.,  411  ff.; 
4te  Aufl.,  438  ff. 


SENSATIONS   OF   TASTE  AND  SMELL.  53 

Z  WAARDEMAKER  :  A.  Die  Bestimmung  der  Geruchscharfe,  Berliner 
kiln.  Wochenschrift,  XXV.,  1888,  No.  47,  950  (abstract  of  the 
same,  British  Med.  Journal,  1888,  ii.  1295);  also  Lancet, 
London,  1889,  i.  1300. 

B.  Compensation  von  Geriichen  mittelst  des  Doppelreichmessers, 
Fortschritte  der  Medicin,  VII.,  1889,  721  if. 

C.  Sur  la  norme  de  1'acuite   olf active  (olfactie),  Archives  neer- 
landaises,  XXV,,  131-148. 

D.  La  niesure  des  sensations  olf  actives  et  Tolfactometre,  Revue 
scientifique,    1889,   ii.   810-812.      Extract   from  the  Archives 
neerlandaises. 


54          LABORATORY   COURSE  IN  PSYCHOLOGY. 


CHAPTER  IV. 
Sensations  of  Hearing. 

-«  JN  these  experiments  a  little  knowledge  of  the  physics  of 
sound  is  presupposed  —  as  much  as  would  be  given  in  an 
elementary  course  in  physics.  A  very  little  knowledge  of 
musical  notation  is  also  required,  but  hardly  more  than 
everybody  has.  No  special  musical  skill  is  needed  except 
in  Exs.  70  and  93  b.  It  is  also  the  author's  belief  that  most 
persons  calling  themselves  "  unmusical,"  however  truly 
they  may  rate  themselves  as  performers,  are  very  much 
in  error  as  to  their  ability  to  discriminate  musical  sounds. 
The  greatest  difficulty  in  some  of  these  experiments  will  be 
found  in  the  continuous  intrusion  of  outside  sounds,  and 
some  even  may  have  to  be  tried  at  night. 

SOUNDS  IN  GENERAL. 

61.  Minimal  Sounds,  a.  Experiment  in  a  large  room  as 
free  as  possible  from  noise.  Let  the  subject  be  seated  with 
his  side  toward  the  experimenter,  his  eyes  closed,  and  his 
ear  upon  the  other  side  plugged  with  cotton.  Let  the 
experimenter  then  find  what  is  the  greatest  distance  at 
which  the  subject  can  still  hear  the  tick  of  a  watch  held 
at  the  level  of  his  ear  and  on  the  prolongation  of  the  line 
joining  the  two  ears.  This  is  easily  done  with  sufficient 
accuracy  by  drawing  a  chalk  line  on  the  floor,  marking  off 
feet  or  meters  and  fractions  upon  it,  and  estimating  by  eye 
the  point  of  the  line  directly  under  the  watch.  Try  several 
times  for  each  ear,  both  when  the  watch  is  being  brought 


SENSATIONS   OF  HEARING.  55 

toward  the  ear  and  when  it  is  being  carried  away.  The 
experimenter  should  from  time  to  time  cover  the  watch  with 
his  hand  to  discover  whether  or  not  the  subject  really  hears, 
or  is  under  illusion.  For  normal  ears  the  distance  found 
may  vary  from  2.5  m.  to  4.5  m.,  and  may  even  rise  to  as 
much  as  9  m. 

b.  The  subject  should  notice  in  this  experiment  the  very 
marked  intermittences  of  the   sound  when  just  upon  the 
limit  of  audibility.     It  will  for  a  few  seconds  be  heard  d^. 
tinctly,  and  a  few  seconds  later  will  as  distinctly  not  be 
heard. 

c.  Faint  sounds  are  apt  to  be  underestimated.     Place  a 
sounding  tuning-fork   on  the  head  and  let  the  sound  die 
away  to  almost  complete  extinction ;  then  remove  it.     The 
drop  to  complete  silence  will  often  seem  larger  than  the 
apparent  intensity  of  the  tone  would  justify. 

On  a,  von  Bezold,  -4;  on  6,  Urbantschitsch,  A*  Lange;  Miinster- 
berg,  A\  on  c,  Stumpf,  I.,  388,  who  quotes  from  Fechner. 

62.  Discriminative  Sensibility  for  Intensity  of   Sounds. 
Exact  experiments  on  this  topic  are  difficult  to  make,  be- 
cause of  the  very  great  difficulty  of  determining  objectively 
the  intensity  of  the  sounds  used.     A  rough  determination 
can  easily  be  made,  however,  with  the  sound  pendulum  (see 
chapter   on    apparatus).      Choose   a  medium   sound  as   a 
standard,  and  by  the  Method  of  Just  Observable  Difference 
explained  under  Ex.  24,  find  a  sound  that  is  just  recogni- 
zably different  from  it.     The  discriminative  sensibility  is 
very  much  finer,  apparently,  when  the  question  is  not  one 
of  recognizing  a  difference,  but  of  locating  a  sound  as  right 
or  left  of  the  median  plane.     Cf.  Ex.  101  and  Eayleigh. 

Wundt,  3te  Aufl.,  L,  364  ff. ;  4te  Aufl.,  I.,  360'ff. ;  Stumpf,  I.,  345  ff. 

63.  Auditory  Fatigue,     a.  Cause   an   assistant  to  strike 
once   with   a  hammer   on  the  floor,  or  to  clap  his  hands. 


56          LABORATORY  COURSE  IN  PSYCHOLOGY. 

With  the  ears  open  a  single  sound,  or  at  most  a  single  sound 
and  transient  echoes  are  heard.  If,  however,  the  ears  are 
kept  closed  with  the  fingers  till  half  a  second  or  more  after 
the  stroke  (the  time  may  easily  be  fixed  by  rapid  counting), 
the  fainter  echoes  will  be  heard  on  the  opening  of  the  ears, 
like  a  new  stroke.  In  the  first  case,  fatigue  from  the  origi- 
nal sound  deadens  the  ears  to  the  fainter  echoes,  though 
they  may  still  be  heard  by  attentive  listening;  in  the 
second  case  they  are  more  strongly  heard  because  the  closed 
ears  are  unfatigued.  The  sound  produced  by  the  simple 
opening  of  the  ears  without  any  objective  stroke  will  be  less 
if  the  finger  is  not  put  into  the  ears,  but  presses  the  tragus 
back  upon  the  opening. 

b.  Strike  a  tuning-fork,  press  the  stem  firmly  upon  the 
mastoid  process,  or  the  crown  of  the  head,  and  hold  it 
there   till  the   tone   is   no   longer  heard.     Then   instantly 
remove  it,  and  after  a  second  or  two  replace  it  upon  the 
same   spot,  taking   pains  to  press  no  harder  than  before. 
The  fork  will  be  heard  again  sounding  faintly.     The  experi- 
ment may  not  succeed  at  first,  but  a  few  trials  should  not 
fail  to  show  the  effect. 

c.  Insert  in  the  openings  of  the  ears  the  ends  of  a  rubber 
tube.     Strike  a  tuning-fork  and  set  it  upon  the  tube  at  such 
a  point  that   it   sounds    equally  intense  to  the  two  ears. 
The  sound  will  then  probably  appear  to  be  located  in  the 
head  midway  between  the  ears  —  at  least  not  nearer  one 
than  the  other.     After  a  few  seconds  strike  the  tuning-fork 
again,  pinch  the  tube  on  one  side,  say  the  left,  so  as  to  shut 
off  the  sound  from  the  ear  on  that  side,  set  the  tuning-fork 
at  the  proper  place  on  the  tube  and  keep  it  there  till  the 
sound  has  become  rather  faint.      Then  allow  the  pinched 
tube  to  open,  and  notice  that  the  sound  is  now  stronger  on 
the  left  than  the  right  and  apparently  located  on  the  left. 
Try  the  experiment  in  reverse  form,  pinching  the  tube  on 
the  right. 


SENSATIONS   OF  HEARING.  57 

Cf.  later  experiments  on  the  analysis  of  compound  tones 
by  the  fatigue  method,  Ex.  89  e. 

Stumpf,..!.,  360-363.  On  a,  Mach;  on  6,  Corradi;  on  c,  Urbant- 
schitsch,  B. 

64.  Inertia  of  the  Auditory  Apparatus,  a.  Inertia  tend- 
ing to  keep  the  auditory  apparatus  out  of  function  can  be 
demonstrated  as  follows.  Place  the  ends  of  a  rubber  tube 
in  the  ears,  and  set  upon  the  middle  of  it  a  low  tuning-fork 
sounding  as  faintly  as  possible.  Notice  that  the  sound  does 
not  reach  its  maximum  intensity  for  an  appreciable  length 
of  time  ;  if  the  fork  is  barely  audible,  this  may  be  as  much 
as  a  second  or  two.  Be  careful  not  to  increase  the  pressure 
of  the  fork  upon  the  tube  after  first  setting  it  on,  for  that 
will  produce  an  objective  strengthening  of  the  tone ;  and 
allow  an  interval  of  several  seconds  between  the  tests  so 
that  the  auditory  apparatus  may  again  come  completely  to 
rest.  A  tuning-fork  that  will  preserve  these  minimal  vibra- 
tions for  some  seconds,  and  complete  freedom  from  distract- 
ing noises,  will  be  found  necessary  for  success. 

b.  Inertia  tending  to  keep  the  auditory  apparatus  in  func- 
tion (positive  auditory  after-images)  can  be  demonstrated  as 
follows.  Fasten  upon  the  front  of  a  rather  solid  pendulum 
a  small  tuning-fork,  so  that  it  shall  project  forward  at  right 
angles  to  the  pendulum  bar  and  the  tines  of  the  fork  shall 
be  vertically  one  above  the  other.  On  the  three  arms  of  a 
Y-tube  attach  three  pieces  of  small  rubber  tubing,  say  quar- 
ter inch  outside  measurement.  Those  fitting  on  the  upper 
arms  of  the  Y  should  be  of  the  same  length,  that  fitting  upon 
the  stem  may  be  of  any  convenient  length.  Insert  the  free 
end  of  the  last  mentioned  tube  in  the  outer  passage  of  the 
ear,  and  hold  the  tips  of  the  other  tubes  about  half  an  inch 
apart,  open  end  upward,  in  such  a  way  that  the  tip  of  the 
tuning-fork,  as  the  pendulum  swings,  will  pass  close  over 
them.  Strike  the  fork  with  a  small  rubber  hammer  as  the 


58          LABORATORY  COURSE  IN  PSYCHOLOGY. 

pendulum  swings  and  notice  the  sound  produced  by  the 
fork  as  it  passes  the  ends  of  the  tube.  If  a  single  continu- 
ous sound  is  heard,  separate  the  tubes  a  little ;  if  a  double 
sound  is  heard,  bring  them  together ;  and  thus  by  shifting 
them  back  and  forth  find  the  place  where  the  sounds  just 
fuse  into  one.  The  auditory  disturbance  occasioned  by  the 
first  pulse  of  sound  outlasts  the  interval  between  the  two, 
and  blends  with  the  second.  Move  the  pendulum  slowly 
over  the  end  of  one  tube  and  then  of  the  other,  meantime 
pinching  the  tube  over  which  the  fork  is  sounding,  to  con- 
vince yourself  that  the  tone  is  not  heard  at  substantially 
the  same  instant  in  both  tubes.  It  is  possible  from  the 
rate  of  the  pendulum  and  the  separation  of  the  tubes  to  find 
approximately  the  length  of  time  through  which  the  sensa- 
tion persists. 

c.  Sometimes  it  is  possible  to  get  more  lasting  after-im- 
ages and  even  those  that  are  recurrent.  Try  with  a  tuning- 
fork  struck  and  held  a  few  seconds  before  one  ear.  Stop 
the  fork  by  touching  it,  without  removing  it  from  the  ear. 
The  after-image  is  not  very  easy  to  observe;  the  lowest 
degree  of  it  seems  to  be  the  transforming  of  faint  outer 
noises  into  something  qualitatively  like  the  tone  heard,  or 
perhaps  a  selection  of  certain  of  those  noises.  The  usual 
interval  between  the  stimulus  and  the  after-image  is  under 
fifteen  seconds.  The  number  of  recurrences  of  the  after- 
image differs  in  different  subjects ;  for  Stumpf,  they  seem 
to  come  by  preference  in  the  unstimulated  ear. 

Stumpf,  L,  211  ff,  278;  Urbantschitsch,  C.  For  methods  of  dem- 
onstration permitting  more  accurate  measurement  of  the  persistence 
of  tone,  see  Urbantschitsch,  C,  and  Mayer,  A. 

65.  Noise.  Whether  or  not  there  is  a  distinctive  sensa- 
tion of  noise  different  from  that  of  a  mass  of  short,  disso- 
nant, and  irregularly  changing  tones,  is  yet  under  debate. 
A  little  attention  to  the  noises  constantly  occurring,  espe- 


SENSATIONS    OF  HEARING.  59 

eially  to  their  pitch,  will  easily  convince  the  observer  that  a 
tonal  element  is  present.  This  is  striking  when  resonators 
(cf.  notes  on  apparatus  for  simultaneous  tones)  are  used, 
for  they  pick  out  and  prolong  somewhat  the  tones  to  which 
they  correspond,  but  they  are  not  indispensable.  On  the 
other  hand,  attention  to  musical  tones  will  often  discover 
the  presence  of  accompanying  noises. 

Wundt,  3te  Aufl.,  L,  420;  4te  Aufl.,  I.,  447  f ;  Stumpf,  II.,  497- 
515;  Brucke;  Exner;  Mach,  B,  117. 

66.  Silence.     When  circumstances   promise   absence   of 
external  sounds,  notice  that  many  are   still   present  and 
distinct,  though  faintly  heard.     Notice  also  the  pitch  and 
changing  character  of  the  subjective  sounds  to  be  heard. 
Our  nearest  approach  to  the  experience  of  absolute  stillness 
is  this  mass  of  faint  inner  and  outer  sensations. 

Preyer,  A,  67-72;  Stumpf,  L,  380  ff. 

SINGLE  AND  SUCCESSIVE  TONES. 

67.  Highest  Tones.     With  the  apparatus  at  hand  for  the 
purpose,  find  what  is  the  highest  audible  tone  ;  i.  e.,  if  the 
cylinders  are  used,  the  shortest  cylinder  which  still  gives  a 
ringing  sound  when  struck  with  the  hammer,  or  if  the  whis- 
tle is  used,  the  closest  position  of  the  plunger  at  which  a 
tone  can  still  be  heard  beside  the  rush  of  air.     If  a  number 
of  persons  are  tested,  it  is  not  improbable  that  some  will  yet 
hear  the  tone  after  it  has  become  inaudible  for  the  rest. 

Same  references  as  Ex.  68. 

68.  Lowest  Tones.     If  low-pitched  tuning-forks  or  other 
vibrators  are  at  hand,  find  what  is  the  slowest  rate  of  vibra- 
tion that  can  yet  be  perceived  as  a  tone.     In  some  physio- 
logical laboratories  electric  tuning-forks  or  interrupters  may 
be  found  that  have  vibration  rates  of  twenty-five  per  second. 
Low  tones  can  be  heard  from  these,  though  they  have  many 


60          LABORATORY  COURSE  IN  PSYCHOLOGY. 

overtones.  The  latter  can  be  partly  damped  by  touching  the 
tines  midway  of  their  length  with  the  finger,  and  partly 
avoided  by  bringing  the  ear  not  to  the  free  end,  but  to  a 
point  somewhat  nearer  the  handle.  The  determination  of 
the  lower  limit  of  audible  pitch  is  difficult  and  uncertain 
because  of  the  great  difficulty  which  observers,  even  those 
of  trained  ear,  find  in  distinguishing  these  lowest  tones  from 
the  next  higher  octaves.  The  general  character  of  these 
deep  tones  can  be  demonstrated  with  sufficient  clearness 
upon  the  contra  octave  (Ci-C)  of  a  church  organ,  if  one  is 
accessible  and  tuning-forks  are  lacking. 

Von  Bezold,  B;  Wundt,  3te   Aufl.,  I.,  423;  4te  Aufl.,  I. ,  450; 
Preyer,  A  and  D;  Stumpf,  L,  263,  II.,  551. 

69.   Some  Characteristics  of  High  and  Low  Tones. 

a.  High  tones  are  smoother  than  low  tones.     This  is  clear 
with  almost  all  tones  used  in  music,  and  particularly  so  with 
those  of  reed  instruments.     The  roughness  of  low  tones  is 
largely  due  to  the  beating  of  their  partials  among  them- 
selves (see  Exs.  86  ff.  and  79  ff.)  and  even  with  the  funda- 
mental tones ;  the  high  tones  having  fewer  audible  partials 
are  freer  from  it.     Play  the  scale  of  any  instrument  from  its 
lowest  to  its  highest  tone,  or  sing  the  ascending  scale.    The 
difference  of  roughness  is  observable  also  with  simple  tones, 
but  only  at  lower  pitches,  and  is  even  there  less  marked. 

b.  In  spite  of  the  generally  accepted  fact  that  high  tones 
produce  a  more  intense  sensation  than  low  tones  of  equal 
physical  energy,  high  tones  are  more  readily  suppressed  by 
stronger  lower  tones  than  vice  versa.     Place  an  ordinary 
clock  at  a  distance  of  a  few  feet  and  hold  close  before  the 
ear  a  watch.     When  the  watch  is  near  the  ear  all  the  ticks 
will  be  heard.     As  it  is  gradually  removed,  a  position  can 
be  found  where  the  watch-tick  that  coincides  with  the  clock- 
tick  will  be  suppressed.     When  both  make  an  equal  number 


SENSATIONS    OF  HEARING.  61 

of  ticks  to  a  second,  and  one  gains  a  little  on  the  other, 
there  will  occur  periods  in  which  no  watch-ticks  are  heard, 
and,  alternating  with  them,  periods  in  which  all  are  heard. 
If  the  watch  beats  oftener  than  the  clock  and  both  run  at 
the  same  rate,  a  single  watch-tick  will  be  lost  at  regular  in- 
tervals. When  the  clock  is  removed,  all  the  ticks  of  the 
watch  can  easily  be  heard  at  the  distance  used.  The  phe- 
nomenon can  be  observed  when  the  watch  is  on  the  opposite 
side  of  the  head  from  the  clock.  To  demonstrate  weakness 
of  high  tones  in  suppressing  lower  tones,  sound  together  a 
large  and  a  small  tuning-fork  on  their  resonance  cases,  e.  g., 
c  and  c",  a'1 ',  or  #",  sounding  the  first  very  faintly  and  the 
second  as  loudly  as  possible.  The  first  will  still  be  heard 
even  when  the  second  is  brought  close  to  the  ear.  In  this 
connection  compare  the  difficulty  of  analyzing  the  compound 
tones  in  Exs.  86  if.,  also  Exs.  83  b  and  84. 

c.  Some  high  tones  are  particularly  strengthened  by  the 
resonance  of  the  outer  passage  of  the  ear.     These  generally 
lie  between  c4  and  c5,  and  give  to  the  tones  of  this  octave  a 
superior  strength  and  ear-piercing  quality.     They  may  be 
demonstrated  easily  with  a  small  piston  whistle.     Find  by 
adjustment  of  the  piston  the  point  at  which  the  tone  is  most 
piercing.     Insert  in  the  outer  ends  of  the  ear-passages  bits 
of  rubber  tubing  half  an  inch  long  (which  will  change  the 
resonance  of  the  passages,  making  them  responsive  to  a 
lower  tone)   and  sound  the  whistle  again.     The  piercing 
quality  will  be  gone  and  the  tone  appear  decidedly  weaker. 
Remove  the  bits  of  tubing  and  sound  the  whistle  as  before ; 
the  original  quality  and  intensity  reappear. 

d.  Very  closely  associated  with  the  pure  tonal  sensations 
are  certain  of  a  spatial  quality.     Compare  in  this  respect 
the  sensations  of  the  tones  observed  in  c  above ;  or,  better 
still,  those  of   Ex.  67  with  those  of  Ex.  68,  or  any  other 
deep  tones.     Play  the  scale  through  the  complete  compass  of 
any  instrument,  keeping  this  quality  in  mind. 


62          LABORATORY  COURSE  IN  PSYCHOLOGY 

e.  Under  certain  conditions,  low  tones  seem  to  be  located 
in  the  head,  high  tones  outside  of  it.  Close  the  ears  with 
the  fingers  and  have  an  assistant  strike  a  low  tuning-fork 
(e.g.,  50  vibrations  per  sec.),  and  set  the  stem  of  it  upon  the 
crown  of  the  head ;  notice  the  location.  Try  the  same 
with  a  high  fork. 

/.  The  emotional  shading  of  tones  changes  with  their 
pitch.  Recall  the  descriptive  terms  used  :  Deep,  low, 
bright,  sharp,  acute.  Play  the  scale,  and  judge  of  the  ap- 
propriateness of  these  terms  to  match  the  shades  of  feeling 
that  mark  the  tones  of  low,  middle,  and  high  pitch,  distin- 
guishing those  that  refer  to  pitch  from  those  enumerated 
in  Ex.  90,  which  refer  to  timbre. 

Stumpf,  L,  202-220,  II.,  56-59,  227;  also  Mach,  B,  120  ff.  On  6, 
Mayer,  7*;  on  c  and  /,  Helmholtz,  116,  179,  and  69  ff.;  on  d,  James, 
II. ,  134  ff.;  on  e,  Kessel. 

70.  Eecognition  of  Absolute  Pitch,  a.  This  experiment 
gives  accurate  results  only  with  those  of  very  decided 
musical  skill,  but  it  may  be  tried  with  any  subject  that 
knows  the  names  of  the  notes.  Strike  various  notes  in  dif- 
ferent parts  of  the  scale  of  the  instrument  and  require  the 
subject  to  name  the  note  given.  Record  the  note  struck 
and  the  subject's  answer.  He  should  be  seated  with  his 
back  toward  the  experimenter,  or  should  keep  his  eyes 
closed. 

b.  Pitch  differences  in  the  perceptions  of  the  two  ears. 
The  same  tone,  heard  first  with  one  ear  and  then  with  the 
other,  seems  to  many  observers,  even  professional  musicians, 
somewhat  different  in  pitch.  Take  two  small  rubber  tubes 
of  equal  size  and  length  (e.  g.,  quarter  inch  tubes,  two  feet 
long),  place  an  end  of  one  in  the  right  ear,  an  end  of  the 
other  in  the  left,  and  bring  the  free  ends  near  together  on 
the  table.  Then  have  an  assistant  strike  a  tuning-fork  and 


SENSATIONS    OF  HEARING.  63 

present  it  alternately  to  the  ends  of  the  tubes.  The 
difference  between  the  two  ears  is  said  to  vary  more  or  less 
from  day  to  day  and  to  be  different  in  amount  for  tones  of 
different^  pitch.  Such  differences  may  be  observed  by  the 
unmusical. 

Stumpf,  I.,  305-313,  also  II.,  index,  Hohenurteile,  for  experiments 
on  trained  musicians;  von  Kries,  J5;  on  6,  Stumpf,  II.,  319  f. 

71.  Just  Observable  Difference  in  Pitch.    Test  as  follows 
with  the  set  of  mistuned  forks.     Let  the  subject  pick  out 
from  the  mistuned  forks  that  which  sounds  to  him  just 
noticeably  different  from  the  normal  fork,  striking  and  hold- 
ing them  successively  (never  simultaneously)  over  a  reson- 
ance bottle.     If  all  of  them  seem  more  than  just  observably 
different,  let  him  put  the  riders  on  the  one  that  is  next 
higher,  and    gradually   lower  the   pitch  by  sliding  them 
toward  the  ends  of  the  fork  till  the  two  forks,  heard  suc- 
cessively, are  just  different  and  no  more.     The  experimenter 
<may  then  determine  the  error  of  the  subject  in  vibrations 
per  second  approximately  by  counting  the  number  of  beats 
produced  by   the   forks   when   sounded   together.     If  the 
number  of  beats  per  second  is  less  than  2  or  more  than  6, 
it  will  be  best  to  get  the  difference  in  pitch  with  some  other 
of  the  forks  first,  so  as  to  avoid  too  slow  or  too  rapid  count- 
ing, and  from  that  to  arrive  at  the  difference   from   the 
standard  fork.     Repeat  the  test  several  times,  sometimes 
sounding  the  standard  fork  first,  and  sometimes  that  to  be 
compared  with  it,  and  average  the  result.     Take  care  to 
avoid  fatigue.     This  experiment  will  not  be  refined  enough 
for  testing  those  of  keen  musical  ear. 

Preyer,  A,  26  ff.,  D,  64;  Stumpf,  I.,  296-305;  Luft. 

72.  Differences  in  Pitch  that  are  Just  Eecognizable  as 
Higher  or  Lower.     It  is  easier  to  recognize  a  difference 
than  to  tell  its  direction.     Experiment  as  in  Ex.  71,  but 


64          LABORATORY   COURSE  IN  PSYCHOLOGY. 

require  the  subject  this  time  to  pick  out  and  adjust  a  fork 
that  is  just  observably  sharper  or  natter  than  the  standard. 

Preyer,  A,  28,  36.     For  experiments  on  extremely  unmusical  sub- 
jects, see  Stumpf,  L,  313-335. 

73.  Number  of  Vibrations  Necessary  to  Produce  a  Sensa- 
tion of  Pitch.     Arrange  an  apparatus  for  blowing   soap- 
bubbles  with  a  mixture  of  hydrogen  and  air.     Blow  bubbles 
of  different  sizes  and  touch  them  off  with  a  match,  either  in 
the  air,  or  (if  proper  precaution  is  taken  to  prevent  the  igni- 
tion of  the  mixed  gases  in  the  vessel  and  any  resonance  in 
the   pipe),  while   still   hanging.     The   explosion   of  these 
bubbles  is  supposed  to  produce  a  single  sound  wave.     The 
pitch  of  the  sounds  produced  cannot  be  accurately  given, 
but  the  report  of  the  large  bubbles  is  distinctly  deeper  than 
that  of  the  small  ones. 

Briicke;  Cross  and  Maltby;  Herroun  and  Yeo. 

74.  The  Apparent  Pitch  of  Tones  is  Affected  by  their^ 
Quality.     Tones  of  dull  and  soft  character  seem  lower  in 
pitch  than  those  that  are  brighter  and  more  incisive.     Re- 
quire the  subject  to  pick  out  on  some  stringed  or  reed  instru- 
ment the  tone  corresponding  to  that  produced  by  blowing 
across  the  mouth  of  a  medium-sized  bottle.     Too  low  a  note 
at  first  will  generally  be  chosen,  at  least  by  those  without 
special  musical  training.     The  tones  should  be  sounded  suc- 
cessively, not  at  the  same  time,  during  the  test.     Afterward 
they  may  be  sounded  together,  and  the  pitch  of  the  bottle 
determined  approximately  by  finding  with  which  tone  of 
the  instrument  its  tone  makes  the  slowest  beats  (cf.  Ex. 
79).     It  should  be  remembered,  however,  that  it  will  be 
possible  to  get  beats  also  with  tones  an  octave  lower  and  an 
octave  higher  than  that  corresponding  most  nearly  with  the 
true  pitch  of  the  bottle  tone. 

Stumpf,  I.,  227-247,  especially,  235-245. 


SENSATIONS    OF  11  EARING,  f  65 


75.  Recognition  of  Musical  IntervalsT'^^rffse  a  familiar 
air  to  be  played,  first  in  the  octave  of  c  and  then  in  that  of 
c"  in  the  same  or  another  key.     Even  those  of  no  musical 
training  .will  easily  recognize  that  the  air  (i.  e.,  the  succes- 
sion of  musical  intervals  in  fixed  rhythmical  relations),  is 
the  same  in  both  cases ;  and  any  mistake  or  variation  will 
be  noticed  as  easily  as  if  the  air  had  been  repeated  at  the 
first  pitch.     With  the  unmusical,  however,  the  recognition 
is  often  rather  of  the  rhythm  than  the  intervals ;  try  there- 
fore a  repetition  of  the  air  changing  some  of  the  intervals 
but  preserving  the  original  rhythm.     The  power  of  recog- 
nizing intervals  is  very  much  more   highly  developed  in 
persons  of  musical  training,  but  any  one  that  can  whistle 
a  tune  at  one  pitch  and  repeat  it  recognizably  at  another 
undoubtedly  has  the  rudiments  of  interval  recognition. 

For  exact  methods  of  testing  the  accuracy  of  the  power  of  recog- 
nizing intervals,  see  Preyer,  A,  38-64;  and  Schischmanow,  and  the 
references  given  by  them. 

76.  Pitch  Distances.      Beside   the  interval  relations  of 
tones,  and  overshadowed  by  them  in  musicians,  are  certain 
relations  of  separateness  or  distinctness  or  distance  in  pitch, 
which  do  not  depend  on  the  ratios  of  vibration  rates.     Equa- 
musical  intervals  (i.e.,  intervals  between   tones  that  have 
vibration  rates  in  a  fixed  ratio  to  each  other,  e.g.,  CD  and 
c"  d")  do  not  correspond  to  equal  pitch  distances.     Sound 
the  half-tone  interval  c  c-sharp  through  the  range  of  the  in- 
strument, beginning  in  the  bass  and  ascending.     Notice  the 
increasing  distinctness  and  separation  of  the  tones  as  the 
interval  is  taken  higher  and  higher.     For  the  very  highest 
tones  there  is  probably  a  decrease  of  separateness  agaiD. 
The  difference  is  most   striking,  hovfever,  with   intervals 
smaller  than  those  in  common  use,  e.g.,  with  quarter  or 
eighth  tones.     On  the  harmonical  (cf.  notes  on  apparatus) 
strike  in  succession  the  c-sharp  and  d  keys  in  the  four  lower 


66          LABORATORY   COURSE  IN  PSYCHOLOGY. 

octaves,  beginning  with  the  lowest.  In  this  instrument  the 
osharp  key  is  given  to  another  d,  a  comma,  or  about  one- 
ninth  of  a  tone,  natter  than  the  regular  d  of  the  scale. 

Stumpf,  I.,  247-253;  Lorenz,  and  the  discussion  between  Wundt, 
Stumpf,  and  Engel;  Helmholtz,  264-265;  Miinsterberg,  C. 

77.  The  Effect  of  a  Given  Tone  in  a  Melody  depends  in 
part  on  the  succession  of  tones  in  which  it  stands.     Cause  a 
simple  air,  in  which  the  same  tone  recurs  in  different  suc- 
cessions of  tones,  to  be  played,  and  notice  the  difference 
in  effect  in  the  different  circumstances,  or  simply  play  the 
ascending  and  descending  scales. 

Mach,  B,  130-131. 

78.  Tones  that  Vary  Irregularly  in  time  and  in  pitch  are 
unpleasant.     Test  with  a  piston  whistle. 

SIMULTANEOUS  TONES. 

79.  Beats.     When  tones  that  are  different  in  pitch  are 
sounded  at  the   same   time,  they  mutually  interfere,  and 
make  the  total  sensation  at  one  instant  more  intense  and 
the  next  instant  less  intense.     This   regular  variation   in 
intensity  is  called  "  beating."     Exs.  71  and  74,  where  beats 
have  been  used  incidentally,  are  a  sufficient  introduction  to 
them. 

a.  The  rapidity  of  beats  depends  on  the  difference  in  the 
vibration  rates  of  the  beating  tones.  Prepare  two  bottle 
whistles  of  the  same  size,  and  blow  both  at  the  same  time. 
Slow  beats  will  probably  be  heard.  If  not,  pour  a  little 
water  into  one  bottle  (thus  raising  the  pitch  of  its  tone),  and 
blow  as  before.  Continue  adding  water,  a  little  at  a  time, 
till  the  beats  lose  themselves  in  the  general  roughness  of 
the  tone.  Blow  the  bottles  separately  now  and  then  to 
observe  the  increasing  difference  in  pitch.  The  same  may 
be  shown  with  a  couple  of  piston  whistles,  if  they  are  first 


SENSATIONS   OF  REARING.  67 

adjusted  to  unison,  and  then  the  piston  of  one  or  the  other 
is  slowly  pushed  in  or  pulled  out. 

b.  Tones  that  are  a  little  more  or  a  little  less  than  an 
octave  apart  may  give  beats.     Try  with  a  pair  of  octave- 
forks  on  resonance  boxes  or  held  over  resonance  bottles,  one 
of  which  has  been  slightly  lowered  in  pitch  by  weighting 
the   prongs  with  wax  or  a  bit  of  rubber  tubing.     In  this 
case  the  beating-tones  are  the  tone  of  the  lower  fork  and 
the  difference  tone  (see  Ex.  82).     Eepeat  the  experiment  on 
a   reed   instrument.     In  this  case  beats  may  be  heard  be- 
tween the  higher  tone  and  the  first  over-tone  of  the  lower 
(see  Ex.  86). 

c.  The  rate  at  which  the  roughness  of   rapid  beats  dis- 
appears, as  also  the  rate  which  produces  the  greatest  rough- 
ness, differs  with  the  pitch  of  the  beating-tones.     Sound  the 
following  pairs   of  tones  which  have  somewhat  near  the 
same  difference  in  vibration  rates  per  sec.,  namely,  33 ;   and 
observe  that  the  roughness  from  the  beats   decreases  and 
finally  disappears  entirely  at  about  the  fourth  pair ;  V  c", 
cf  d',  e  g,  c  e,  Go,  C  G.     The  a!  and  c"  tuning-forks  give  a 
vanish  of  roughness,  representing  a  rate  of  80-88  per  sec. 

Helmholtz,  159-173;  Stumpf.,  II.,  449-497,  especially  461-465 ; 
Mayer,  A  ;  Cross  and  Goodwin. 

80.  Beats  Betray  the  Presence  of  very  Faint  Tones,  both 
because  the  total  stimulus  is  actually  stronger  in  the  phase 
of  increased  intensity,  and  because  intermittent  stimuli  are 
themselves  more  effective  than  continuous  ones. 

a.  Strike  a  pair  of  beating  tuning-forks,  and  hold  one  at 
such  a  distance  from  the  ear  that  it  is  very  faint  or  quite 
inaudible.     Then  bring  the  other  fork  gradually  toward  the 
ear,  and  notice  the  unmistakable  beats. 

b.  Strike  a  tuning-fork  and  hold  it  at  a  distance,  being 
careful  to  have  the  fork  sidewise  or  edgewise,  not  corner- 


68          LABORATORY  COURSE  IN  PSYCHOLOGY. 

ing,  toward  the  ear.  Rotate  the  fork  one  way  and  the 
other  about  its  long  axis,  and  observe  the  greater  distinct- 
ness of  the  tone,  due  in  this  case  simply  to  its  intermit- 
tence. 

81.  Beats  are  in  general  Attributed  to  the  Tone  that 
Receives  Attention ;  in  the  absence  of  other  determining 
causes,  to  the  louder  tone,  to  the  lower  tone,  or  to  the 
whole  mass  of  an  unanalyzed  compound  tone  (see  intro- 
duction to  Ex.  86). 

a.  Set  two  properly  tuned  resonance  bottles  about  a  foot 
apart  on  the  table.     Strike  two  forks  that  beat,  and  hold 
them  over  the  bottles.     While  both  are  about  equally  in- 
tense, it  is  easy,  by  mere  direction  of  the  attention,  to  make 
the  beats  shift  from  one  to  the  other. 

b.  Turn  one  of  the  forks  an  eighth  of  a  turn  about  its 
long  axis,  which  will  weaken  its   tone,  and  observe  that 
the  beats  seem  to  come  from  the  other  fork.     By  turning 
first  one  fork  and  then  the  other,  the  location  of  the  beats 
may  again  be  made  to  shift  at  pleasure.     If  tuning-forks  on 
resonance  boxes  are  at  hand  they  may  be  used,  and  the  tone 
of  one  weakened  by  covering  the  opening  of  the  box  with  a 
bit  of  cardboard. 

c.  Warm  the  cr  fork  in  any  convenient  way  (holding  it 
clasped  in  the  hand  will  do).     This  will  flatten  it  some- 
what.   Strike  it  and  the  c"  fork,  and  press  the  stems  of  both 
on  the  table  at  the  same  time ;  or,  better,  on  the  sounding- 
board  of  the  sonometer.     Observe  that  the  beats  seem  to 
come  from  the  cr  fork  unless  it  is  very  faint. 

d.  Tune  a  string  of  the  sonometer  so  that  its  third  partial 
(or  corresponding  harmonic)  beats  slowly  with  the  c"  fork. 
(On  partials  and  harmonics  cf.  Exs.  86-89.)     Strike   the 
tuning-fork,  and  hold  it  over  a  resonance  bottle,  or  press  its 
stem   against   the   table  at  arm's  length  from  the   string. 
Then  pluck  the  string  and  attend  to  its  tone ;  the  beats  may 


SENSATIONS    OF  HEARING.  .     69 

seem  to  affect  the  whole  compound  tone  of  the  string. 
But  this  will  not  happen  if  the  tone  of  the  string  is  an- 
alyzed, or  if  the  attention  is  directed  to  the  fork.  The 
same  may  be  tried  on  the  piano  by  picking  out  from  the 
mistimed  c"  forks  one  that  beats  slowly  with  c"  on  the 
piano.  Strike  the  /  key  and  hold  it  down ;  strike  the  fork, 
and  observe  the  beats  as  before.  Cf.  Ex.  69  a. 
Stumpf,  II.,  489-497. 

82.  Combination  Tones:  Difference  Tones.1  When  two 
tones  are  loudly  sounded  at  the  same  time  they  produce  by 
their  combination  other  tones,  one  of  a  pitch  represented  by 
the  difference  of  the  vibration  rates  of  the  two  original  or 
generating  tones,  and  one  of  a  pitch  corresponding  to  their 
sum.  The  existence  of  the  summation  tones  has  been  dis- 
puted, and  they  are  hard  to  hear.  The  difference  tones, 
however,  are  easy  to  hear,  at  least  when  they  are  consider- 
ably lower  in  pitch  than  the  generators,  when  the  latter  are 
loud  and  sustained,  and  when  they  make  a  consonant  in- 
terval —  though  the  last  is  not  essential.  A  loud  difference 
tone  may  itself  take  the  part  of  a  generator  and  produce  yet 
another  difference  tone  —  a  difference  tone  of  the  second 
order  —  and  so  on,  though  difference  tones  of  higher  orders 
are  heard  with  difficulty  even  by  skilled  observers.  Differ- 
ence tones  are  hard  to  hear  on  the  piano  and  similar  stringed 
instruments  because  of  the  rapid  decline  in  the  strength  of 
the  generators.  The  difference  tones  are  sometimes  called 
Tartini's  tones,  after  an  early  observer  of  them. 

a.  Repeat  Ex.  79  &,  continuing  to  pour  water  into  one  of 
the  bottles  till  the  difference  tone  appears.  At  first  the 
roughness  of  the  beats  and  the  difference  tone  may  both  be 

1  Konig  distinguishes  between  "  difference  tones "  and  "beat  tones."  Both 
tones,  however,  generally  have  the  same  pitch,  and  the  older  term  for  them  has 
here  been  retained  ;  strictly  speaking,  however,  the  "difference  tones  "  heard  in 
these  experiments  are  "  beat  tones." 


70          LABORATORY   COURSE  IN  PSYCHOLOGY. 

heard  at  once.  Try  the  same  with  the  piston  whistles,  first 
setting  them  at  unison,  and  then  slowly  pushing  the  piston 
of  one  in  or  out  while  blowing  rather  hard.  The  beats  will 
almost  immediately  give  place  to  a  low  difference  tone 
which  may  be  heard  ascending  through  several  octaves 
before  becoming  indistinguishable  from  the  generators. 
The  double  warning  whistles  used  by  bicyclists  give  a  fine 
difference  tone,  to  which  indeed  they  owe  their  deep  and 
locomotive-like  quality. 

b.  Difference  tones  are  strong  on  reed  instruments.     Press 
the  adjacent  white  keys  of  a  parlor  organ,  or  the  harmonical, 
by  twos,  beginning  at  c  and  going  up  a  couple  of  octaves. 
If  there  is  difficulty  in  hearing  the  difference  tone,  sound 
the  upper  tone  intermittently  and  listen  for  the  difference 
tone  at  the  instant  of  pressing  the  key. 

c.  Sound  c"  and  d"  which  should  give  C  as  a  difference 
tone  (594—528=66).     Sound  also  d"  and  e"  which  should 
give  the  same  (660—594=66).     If,  however,  the  tuning  is 
inexact,  as  it  is  intentionally  in  the   tempered   tuning  of 
keyed  instruments,  these  difference  tones  will  be  somewhat 
different  and  may  be  heard  to  beat  with  each  other  when  c", 
d"  and  e"  are  sounded  at  once.     Notice  that  these  beats  are 
not  heard  when  the  tones  are  sounded  in  pairs.     On  the 
harmonical  this  difference  may  be  brought  about  by  sound- 
ing one  of  the  tones  flat  by  pressing  its  key  only  a  little 
way   down.     The   same   thing   may  be    shown  with   three 
piston  whistles  blown  at  once,  by  a  little  careful  adjustment 
of  the  pistons. 

d.  In  the  case  of   reed  instruments  the  difference  tones 
probably  owe  part  of  their  intensity  to  the  vibrations  of  the 
air  in  the  wind  chest.     When  two  whistles  are  blown  by  one 
person  something  of  the  same  kind  may  happen.     In  order 
to  make  a  clean  experiment,  have   the  whistles  blown  by 
two  assistants,  or  observe  the  difference  tones  from  tuning- 
forks. 


SENSATIONS   OF  HEARING.  71 

e.  The  location  of  difference  tones.  The  location  of  these 
tones  is  sometimes  influenced  by  the  location  of  their 
generators,  but  under  favorable  circumstances  they  seem  to 
arise  in  the  ears  or  even  in  the  head.  This  is  strikingly  the 
case,  both  for  the  blower  and  the  listeners,  with  the  differ- 
ence tones  produced  with  the  piston  whistles.  Cf.  Ex.  69  e. 

Helmholtz,  152-159;  Stumpf,  II.,  243-257;  Konig;  Preyer,  C  and 
D;  Hermann. 

83.  Blending  of  Tones.  The  degree  to  which  tones  blend 
with  one  another  differs  with  the  interval  relation  of  the 
tones  taken.  It  is,  according  to  Stumpf,  greatest  with  the 
octave,  less  with  the  fifth,  less  again  with  the  fourth,  slight 
with  the  thirds  and  sixths,  and  least  of  all  with  the  remain- 
ing intervals. 

a.  Try  on  the  instrument  the  extent  to  which  the  tones 
forming  these  intervals  blend,  also  those  forming  intervals 
greater  than  the  octave  :  double  octave,  twelfth,  etc. 

b.  The  blending  in  case  of  the  octave  is  so  complete  under 
favorable  circumstances  as  to  escape  the  analysis  of  trained 
ears.     Use  two  tuning-forks,  one  an  octave  higher  than  the 
other,  on  resonance  cases  or  held   over  resonance  bottles. 
Sound  the  forks,  first  the  higher,  then  the  lower.     For  a 
while  the  higher  fork  will  be  heard  sounding  in  its  proper 
tone,  but  by  degrees  it  will  become  completely  lost  in  the 
lower,  and  a  subject  with  closed  eyes  will  be  unable  to  say 
whether  or  not  it  yet  sounds.    Cf.  Ex.  69  b.    Stop  the  lower 
fork,  or  remove  it  from  its  resonance  bottle,  and  notice  that 
the  higher  is  still  sounding.     Notice  the  change  in  timbre 
(cf .  Ex.  90)  produced  by  the  stopping  of  the  higher  fork  — 
something  like  the  change  from  the  vowel  0  to  the  vowel 
U  (oo). 

On  a,  Stumpf,  II.,  127-218,  especially  135-142;  for  his  experi- 
ments on  the  unmusical  confirming  his  grades  of  blending,  142-173. 
On  6,  Stumpf,  II.,  352-358,  and  Helmholtz,  60-61. 


72          LABORATORY  COURSE  IN  PSYCHOLOGY. 

84.  Analysis  of  Groups  of  Simultaneous  Tones.     Ease  of 
analysis  depends  on  a  number  of  conditions,  among  others 
on  the  following. 

a.  Analysis  is  easier  for  tones  far  distant  in  the  scale. 
Compare  the  ease  of  recognizing  the  sound  of  the  c"  fork 
when  c'  and  c"  are  sounded  together,  with  that  of  recogniz- 
ing cm  when  sounded  with  c'.     Compare  also  the  ease  of 
distinguishing  c'  and  a!  with  that  of  distinguishing  c'  and 
a". 

b.  Analysis  is  made  easier  by  loudness  in  the  tone  to  be 
separated.     Eepeat  Ex.  83  b,  sounding  the  cf  faintly,  the  c" 
strongly.     Little  difficulty  will  be  found  in  keeping  the  lat- 
ter distinct. 

c.  Analysis  is  easier  when  the  tones  make  intervals  with 
little  tendency  to  blend.     Compare  the  ease  of  analysis  of 
cf  c"  and  c'  a'  or  a'  c" .     Also  notice  that  the  addition  of  d" 
(octave  of  df,  fifth  of  g1 r,  fourth  below  g"}  to  the  chord  g  df 
gr  g"  produces  a  less  striking  change  than  the  addition  of  b' 
(major  third  of  gf,  minor  sixth  below  /')  to  the  same  chord. 

d.  Analysis   is   easier  with   sustained   than  with   short 
chords.     Eepeat  the  last  experiment,  making  the  chords  very 
short,  and  notice  that  the  difference  made  by  inserting  either 
d"  or  V  is  less  marked.     Cf.  also  Ex.  100. 

Stumpf,  II.,  318-361;  also  his  experiments,  362-382. 

85.  The   Lower   Tone  of  a   Chord  Fixes   the  Apparent 
Pitch  of  the  Whole,     a.   Repeat  Ex.  83  #,  and  notice  that 
when  the  c'  fork  is  stopped,  the  tone  appears  to  jump  up- 
ward an  octave  in  pitch  (i.e.,  it  takes  the  pitch  of  the  c"  still 
sounding)  ;  but  when  the  c"  fork  is  removed,  the  quality  of 
the  tone  is  changed,  but  not  its  pitch. 

b.  Strike  the  chord  C  c"  e"  g"  or  G  e'  gr  c" ',  and  compare 
the  effect  upon  the  pitch  of  the  whole  mass  of  tone  pro- 
duced by  omitting  C  or  G  alone  with  that  of  omitting  any 
one  or  all  three  of  the  higher  tones.  See  also  the  function 


SENSATIONS    OF  HEARING.  73 

of  the  lowest   partial  of  a  compound  tone  in  fixing  the 
pitch,  noticed  below. 
Stumpf,  II.,  383-392. 

86.  Compound  Tones.  Almost  all  tones  heard,  and  in- 
deed all  those  used  in  music,  are  not  simple  tones,  but  com- 
pound. The  tone  given  by  the  C  string  of  a  piano  is  made 
up  of  at  least  C,  c,  g,  c',  e'  and  g',  and  generally  other  tones. 
The  lowest  tone  of  the  group  gives  the  pitch  attributed  to 
the  whole,  and  is  known  as  the  fundamental,  the  other  tones 
as  over-tones.  In  another  way  of  naming  them,  the  com- 
ponent tones  are  all  partial  tones  or  partials,  the  fundamen- 
tal being  called  the  first  or  prime  partial,  the  next  higher 
the  second  partial  and  so  on.  The  first  over-tone  is  thus 
the  second  partial  tone,  the  second  over-tone  the  third  partial, 
and  in  general  the  same  tone  receives  as  a  partial  tone  a 
number  one  higher  than  as  an  over-tone.  The  vibration 
rates  of  the  partial  tones  of  a  compound  are  generally  once, 
twice,  three  times,  four  times,  the  rate  of  the  fundamental, 
and  so  on.  In  some  cases,  however,  e.g.,  in  bells  and  tun- 
ing-forks, one  or  more  of  the  partial  tones  may  have  a  vibra- 
tion rate  not  represented  in  this  series,  and  discordant 
with  the  fundamental  tone.  In  what  follows,  the  regular 
series  of  partial  tones  is  meant  except  where  the  contrary 
is  specified. 

Partial  Tones.  If  resonators  are  at  hand,  the  demon- 
stration of  the  partial  tones  will  be  easy.  Sound  on  a 
stringed  or  reed  instrument  the  tones  to  which  the  resona- 
tors are  tuned,  and  notice  that  they  resound  strongly  to 
these  tones  and  less  strongly  or  not  at  all  to  other  tones 
adjacent  in  pitch.  Then  sound  the  tone  to  which  the  lar- 
gest of  the  resonators  is  tuned  (or  a  tone  an  octave  lower), 
and  try  the  resonators  in  succession.  Notice  that  others 
also  resound  (at  their  own  proper  pitch),  thus  betraying 
the  presence  of  the  tones  to  which  they  are  tuned,  and 


74          LABORATORY  COURSE  IN  PSYCHOLOGY. 

thus  the  composite  character  of  the  tone  under  examination. 
Which  resonators  will  "  speak  "  will  depend  on  the  instru- 
ment used  ;  reed  instruments  give  a  long  and  perfect  series, 
piano  and  stretched  wires  a  perfect  series  generally  as  far 
as  the  ninth  or  tenth  partial,  and  stopped  organ-pipes  a 
short  series.  If  difficulty  is  found  in  knowing  when  the 
resonator  is  resounding,  it  will  be  found  useful  to  apply  it 
to  the  ear  intermittently,  alternating,  for  example,  two  sec- 
onds of  application  with  two  seconds  of  withdrawal. 

87.  Partial  Tones  :    Analysis  by  indirect  means,     a.   By 
sympathetic  vibration.     This  succeeds  especially  well  with 
the  piano.     Press  the  c  key  and  hold  it  down  so  as  to  leave 
its  strings  free  to  vibrate  ;  then  strike  the  C  key  forcibly, 
and  after  one  or  two  seconds  release  it.    The  c  strings  will  be 
found  to  be  sounding.     Eepeat,  trying  c-sharp  or  b  instead 
of  c;  they  will  be  found  not  to  respond.     Eepeat  the  experi- 
ment, substituting  </,  c',  e',  g',  or  c" ;  all  will  be  found  to  re- 
spond but  in  lessening  degrees.     Other  keys  between  C  and 
c"  may  be  tried  but  will  be  found  in  very  faint  vibration,  if 
at  all. 

b.  By  beats.  This  will  succeed  best  with  a  reed  instru- 
ment, e.  g.,  a  parlor  organ  or  the  harmonica!.  By  pressing 
the  keys  of  the  instrument  only  a  little  way  down,  any  of  its 
tones  may  be  sounded  a  little  flatter  than  its  true  pitch  and 
so  in  condition  to  beat  with  any  other  tone  having  that  true 
pitch.  Sound  at  this  flattened  pitch  the  over-tones  of  C  in 
succession  while  C  is  sounding,  and  notice  the  slow  beats 
that  result.  For  verification  sound  other  tones  not  over-tones 
of  (7,  and  notice  that  the  beats  when  present  are  much  more 
rapid. 

88.  Partial  Tones  :  Direct  analysis  without  special  appara- 
tus.    The  directions  given  here  apply  to  the  sonometer,  but 
will  be  readily  adaptable  to  any  stringed  instrument  in  which 


SENSATIONS    OF  HEARING.  75 

the  strings  can  be  exposed.  It  is  easier  to  hear  any  partial 
tone  in  the  compound,  if  the  partial  is  first  heard  by  itself, 
and  then  immediately  in  combination  with  the  rest.  On 
strings  this  is  easily  done  by  sounding  the  partials  as 
"harmonics."  Pluck  the  string  near  one  end  (say  about 
one-seventh  of  the  length  of  the  string  from  the  end),  and 
immediately  touch  it  in  the  middle  with  the  finger  or  a 
camel' s-hair  brush.  The  fundamental  will  cease  to  sound 
and  its  octave  (the  second  partial)  will  be  left  sounding,  as 
a  "  harmonic."  With  it  sound  also  other  even-numbered 
partials,  but  less  strongly.  Pluck  as  before,  and  touch  the 
string  at  one-third  its  length ;  the  third  partial  will  now 
sound  out  strongest,  with  the  sixth,  ninth,  etc.,  more  faintly. 
Thus  by  plucking  the  string  and  touching  it  respectively 
at  one  half,  one  third,  one  fourth,  one  fifth,  one  sixth,  one 
seventh,  one  eighth,  one  ninth,  and  one  tenth  its  length  from 
the  end,  the  series  of  tones  corresponding  to  the  2d,  3d,  4th, 
5th,  6th,  7th,  8th,  9th,  and  10th  partials  can  be  heard,  each 
in  large  measure  by  itself.  In  getting  the  higher  "harmon- 
ics "  it  will  be  found  better  to  pluck  nearer  the  end  than 
one  seventh,  and  in  no  case  should  the  string  be  plucked  at 
the  point  at  which  it  is  presently  to  be  touched.  (Cf.  Ex. 
90S.) 

To  hear  the  partial  tones  when  sounding  in  the  compound, 
proceed  as  follows.  Sound  the  required  tone  as  a  "  har- 
monic," and  then  keeping  the  attention  fixed  on  that  tone, 
stop  the  string  and  pluck  it  again,  this  time  letting  it  vibrate 
freely.  The  tone  just  heard  as  a  "  harmonic  "  will  now  be 
heard  sounding  with  the  rest  as  a  partial.  When  the  partial 
is  thus  made  out,  verify  the  analysis  by  touching  the  string 
again  and  letting  the  tone  sound  once  more  as  a  "harmonic." 
Try  in  this  way  for  the  partials  up  to  the  tenth ;  first  for 
the  3d,  5th,  and  7th,  afterward  for  the  6th,  4th,  and  the  2d, 
which  is  the  most  difficult  of  all.  It  is  said  that  analysis 


76          LABORATORY   COURSE  IN  PSYCHOLOGY. 

is  easier  at  night  (not  alone  on  account  of  the  greater  still- 
ness) and  when  one  ear  is  used,  and  that  certain  positions 
of  the  head  favor  certain  partials. 

89.  Partial   Tones :   Direct   analysis  without  apparatus. 
Certain  parts  of  a  compound  tone  are  sometimes  so  sepa- 
rated by  their  dissonance,  intensity,  or  pitch  that  they  stand 
out  with  striking  clearness. 

a.  Strike  a  tuning-fork  on  a  hard  surface,  and  observe  the 
high,  ringing,  dissonant  partials.     They  fade  out  before  the 
proper  tone  of  the  fork,  and  are  heard  best  when  the  fork 
is  not  held  near  the  ear. 

b.  As  the  tone  of  a  string  is  allowed  to  die  away  of  itself, 
different  partial  tones  come  successively  into  prominence. 
Try  with  a  low  piano  string,  keeping  the  key  pressed  down 
while  the  sound  fades,  or  with  the  sonometer.     Something 
of  the  same  kind,  but  less  marked,  happens  in  the  dying 
away  of  a  low  tone  on  a  reed  instrument  when  the  air  is 
allowed  to  run  low  in  the  bellows. 

c.  When  a  tone  is  sounded  continuously  for  some  time  on 
a  reed  instrument  with  one  of  the  keys  clamped  down,  dif- 
ferent partials   come  successively  into  prominence,  either 
through  varying  fatigue  or  the  wandering  of  attention. 

Helmholtz,  36-65;  Stumpf,  II.,  231-243;  see  also  the  index  under 
Obertone  ;  Mach,  A,  58,  B,  127. 

90.  Timbre.     The  peculiar  differences  in  quality  of  tones 
(distinct   from  pitch  and   intensity)  which  are   known  as 
differences  in  timbre  (tone-color,  clang-tint,  Klangfarbe) ,  are 
due  largely  to  differences  in  the  number,  pitch,  and  intensity 
of  the  partial  tones  present.     Compare  in  this  respect  the 
dull-sounding  bottle-tones  or  the  tones  of  tuning-forks  held 
over  resonance  bottles,  and  the  more  brilliant  tones  of  a  reed 
or  stringed  instrument ;  the  first  are  nearly  simple  tones, 
while  the  second  have  strong  and  numerous  over-tones. 


SENSATIONS    OF  HEARING.  77 

a.  Notice  the  difference  in  quality  between  the  tone  given 
by  a  tuning-fork  held  before  the  ear  and  that  given  by  the 
same  fork  when  its  stem  is  pressed  upon  the  table.     In  the 
second  position  the  over-tones  are  relatively  stronger. 

b.  Notice   the   differences  in   quality  in   the   tone  of  a 
string  when  it  is  plucked  in  the  middle,  at  one  third  its 
length  and  at  about  one  seventh.     When  plucked  in  the 
middle,  many  odd-numbered  partials  are  present,  and  the 
even-numbered  partials  are  either  absent  or  extremely  faint, 
and  the  tone  is  hollow  and  nasal ;  when  plucked  at  one  third, 
the  third,  sixth,  and  ninth  partials  are  wanting,  and  the  tone 
is  hollow,  but  not  so  much  so  as  before ;  when  plucked  at 
one  seventh  all  the  partials  up  to  the  seventh  are  present. 
For  their  theoretical  intensities,  cf.  Helmholtz,  79. 

c.  Try  also  plucking  very  near  one  end,  plucking  with 
the  finger-nail  and  striking  the  string  with  a  hard  body,  e.  g., 
the  back  of  a  knife-blade  ;  all  these  bring  out  the  higher  and 
mutually  discordant  partials  strongly,  and  produce  a  .brassy 
timbre. 

Helmholtz,  65-119  ;  Stumpf,  II.,  514-549. 

91.  In  Successive  Chords  the  Whole  Mass  of  Tone  seems 
to  move  in  the  same  direction  as  the  part  that  changes  most. 
Strike  in  succession  the  chords  e'  gr -sharp  l>  e'' ',  a  a!  c" -sharp 
e" ,  or  a  cr  e'  c" ,  a  cr  f  c" .  If  the  attention  is  directed  to  the 
bass  in  the  first  example  and  to  the  alto  in  the  second  the 
whole  mass  of  tone  will  appear  to  descend  in  the  first  case 
and  to  ascend  in  the  second.  If  the  attention  is  kept  on  the 
soprano  part  the  illusion  will  not  appear,  as  also  when  the 
observer  examines  his  sensations  critically.  Cf.  also  Ex.  81 
d,  where  beats  of  a  partial  tone  are  attributed  to  the  whole 
compound  tone. 

Mach,  B,  126-127;  Stuinpf,  II.,  393-395, 


78 


LABORATORY   COURSE  IN  PSYCHOLOGY. 


92.    Simultaneous    Tones   interfere   somewhat  with   one 
another  in  Intensity. 


la. 


a.  Play  the  groups  of  notes  numbered  1,  2,  and  3  and  ob- 
serve the  slight  increase  in  the  apparent  intensity  of  the 
remaining  tones  as  one  after  another  drops  out,  making  1 
sound  like  la,  2  like  2a,  and  so  on.  On  the  piano  it  will  be 
well  to  play  the  notes  an  octave  or  two  lower  than  they 
are  written. 


b.  Play  the  notes  marked  4,  and  notice  that  the  increase 
of  loudness  seems  to  affect  the  note  (highest  or  lowest)  that 
receives  particular  attention,  making  the  effect  in  one  case 
like  4a,  in  the  other  like  4&. 

Mach,  J3,  126;  Stumpf,  II.,  418-423. 

93.  Consonant  and  Dissonant  Intervals,  a.  The  conso- 
nant intervals  within  the  octave  are  the  unison,  octave,  fifth, 
fourth,  major  sixth,  major  third,  minor  third,  and  minor 
sixth.  They  will  be  found  to  decrease  in  smoothness  about 
in  the  order  given.  Try  them  beginning  with  the  octave 
and  at  c,  as  follows  :  c  c',  eg,  cf,ca,  c  e,  c  e-flat,  c  a-flat. 
Try  the  last  four  intervals  also  in  the  octave  of  c"  or  c"' 
and  notice  that  they  are  less  rough  than  when  taken  in  the 


SENSATIONS    OF  HEADING.  79 

octave  of  c.  Any  other  intervals  within  the  octave  are  dis- 
sonant. Try  c  c-sharp,  c  d,  c  b,  c  b-flat,  c  f-sharp.  The 
roughness  is  due  to  beating  partial  tones  and  in  general 
is  greater  when  these  stand  low  in  the  partial  tone  series 
and  are  loud,  and  when  they  lie  within  a  half-tone  of  each 
other.  Work  out  for  the  tones  of  several  of  the  intervals 
the  series  of  partial  tones  up  to  the  eighth.  In  general  the 
extension  of  intervals  into  the  second  octave  (taking  the 
higher  tone  an  octave  higher  or  the  lower  tone  an  octave 
lower)  does  not  change  the  fact  of  consonance  or  dissonance, 
though  it  may  change  the  relative  roughness. 

b.  Those  fitted  by  musical  training  to  pronounce  upon 
questions  of  consonance  and  dissonance  hold  that  dissonance 
can  be  perceived  between  simple  tones  under  conditions  that 
exclude  beats,  and  that  consonance  is  something  more  than 
the  smooth  flowing  of  tones  undisturbed  by  beats.  The 
test  is  easy  to  make.  Hold  tuning-forks  making  the  inter- 
val to  be  tested  one  before  each  ear,  and  if  there  are 
beats,  carry  the  forks  far  enough  away  in  each  direction 
to  make  the  beats  inaudible.  Only  those  of  musical  ear, 
however,  can  pronounce  upon  the  result. 

Helmholtz,  179-197;  Stumpf,  II.,  470,  460;  Wundt,  3te  Aufl.,  I., 
439,  II.,  47  ff ;  Mach,  B,  129-130;  Preyer,  D,  44  ff. 

94.  Consonant  and  Dissonant  Chords.     In  order  to  form 
a  consonant  chord,  all  the  intervals  among  the  tones  must 
also  be  consonant.     The  only  chords  of  three  tones  which 
fulfil  this  condition  within  the  octave  are  represented  by 
the  following :  Major  c  e  g,  cf  a,  c  e-flat  a-flat,  minor  c  e- 
flat  g,  c  f  a-flat,  c  e  a.     Try  these  and  for  comparison  any 
other  chord  of  three  tones  having  c  for  its  lowest  tone. 

Helmholtz,  211  ff.;  Wundt,  3te  Aufl.,  II.,  61,  63  ff. 

95.  Major  and  Minor  Chords.     Compare  the  chords  c"  e" 
g"  and  c"  e"-flat  g".     This  unmistakable  difference  in  effect 


80          LABORATORY  COURSE  IN  PSYCHOLOGY. 

depends  in  part  at  least  on  the  fact  that  in  the  major  chord 
the  difference  tones  of  the  first  order  are  lower  octaves  of  c" 
itself ,  while  in  the  minor  chord  one  difference  tone  is  not 
such  at  all,  and  if  taken  in  the  same  octave  with  the  chord 
would  be  highly  dissonant.  For  the  major  chord,  when 
taken  in  the  octave  of  c" ',  the  difference  tones  are  c  and  c", 
for  the  minor  chord  c  e-flat,  A-flat.  Try  on  a  reed  instru- 
ment the  difference  tones  generated  by  c"  e",  e"  g",  c"  e"-flat, 
e"-flat  g",  first  separately  ;  and  then,  while  c"  and  g"  are 
kept  sounding  strike  e"  and  e"-flat  alternately. 

Helmholtz,  215-217;  Stumpf,  II.,  335,  376  ff.;  Wundt,  3te.  Aufl., 
II.,  61  ff.,  67  ff. 


96.  Cadences.    Modern  music  requires  the  prominence  of 
the  key  note  or  tonic  and  of  the  chord  in  which  it  holds  the 
chief  place  at  the  beginning  of  a  piece  of  music  and  at  the 
end.     The  feeling  of  the  appropriateness  of  this  close,  and 
especially  of  the  succession  of  chords  in  the  cadences  above, 
can  hardly  fail  to  appeal  even  to  the  unmusical. 

Helmholtz,  293. 

97.  The  Absolute  Time  Eelations  of  music  have  much  to 
do  with  its  emotional  effect.     Have  a  familiar  piece  of  music 
played  in  its  proper  time,  then  very  slowly  and  very  rapidly. 


SENSATIONS   OF  BEARING.  8l 


BINAURAL  AUDITION  AND  THE  LOCATION  OF  SOUNDS. 

98.  Unison  Tones  Heard  with  the  Two  Ears.  a.  Strike 
a  pair  of  unison  forks  that  will  sound  equally  loud  and 
vibrate  an  equal  length  of  time,  and  hold  one  before  each 
ear,  three  or  four  inches  away ;  a  single  tone  of  rather  in- 
definite location  will  be  heard.  As  the  forks  are  brought 
nearer,  their  tone  seems  to  draw  by  degrees  toward  the 
median  plane ;  and  when  they  are  very  loud  and  near,  the 
tone  may  seem  to  be  in  the  head.  Return  the  forks  to 
their  first  position  and  then  move  one  a  little  nearer  or  a 
little  farther  away,  and  notice  that  the  sound  moves  to  the 
side  of  the  nearer  fork.  When  the  difference  in  distance 
has  become  considerable  that  fork  alone  will  be  heard. 

b.  Bring  the  forks  again  into  the  positions  last  mentioned 
—  one  near  and  one  far,  (or  better,  place  one  fork  on  a  rub- 
ber tube  one  end  of  which  has  been  inserted  in  the  opening 
of  the  ear  and  hold  the  other  fork  before  the  other  ear), 
and  then  with  the  free  or  more  distant  fork  make    slow 
rhythmical  motions  toward  and  away  from  the  ear,  or  rotate 
the  fork  slowly  about  its  long  axis,  attending  meantime  to 
the  fork  on  the  other  side.     Alternate   variations  in  the 
intensity  of  the  tone  of  this  fork  corresponding  to  the  ap- 
proach and  recession  of  the  other  and  apparently  unheard 
fork  can  be  observed. 

c.  Repeat  b  and  notice  that  when  the  changes  in  intensity 
are  considerable  there  is  a  simultaneous  shifting  of  the  place 
of  the  tone,  towards  the  median  plane  when  the  tone  grows 
stronger,  and  away  when  it  grows  fainter.     These  changes  of 
place  are,  however,  less  marked  than  the  changes  in  intensity 
and  those  accompanying  slight  changes  in  intensity  gener- 
ally escape  observation. 

Schaefer,  J5;  Thompson;  Urbantschitsch,  B. 


82         LABORATORY  COURSE  IN  PSYCHOLOGY. 

99.  Beats  Heard  with  Two  Ears.     a.    Operate  as  in  Ex. 
98  a,  with  forks  beating  three  or  four  times  a  second. 

b.  Try  with  a  pair  of  very  slow  beating  forks  (once  in 
two  or  three  seconds).     Notice  a  shifting  of  the  sound  from 
ear  to  ear  corresponding  to  the  rate  of  beating. 

c.  Try  again  with  a  pair  of  rapid  beating  forks  (twenty  or 
thirty  a  second),  and  notice  that  the  beats  are  heard  in  both 
ears. 

Schaefer,  A,  B,  and  O;  Thompson;  Cross  and  Goodwin. 

100.  Difference  of  Location   Helps   in  the  Analysis  of 
Simultaneous  Tones.     Compare   the   ease  with  which  the 
tones  of  a  pair  of  octave  forks  are  distinguished  when  the 
forks  are  held  on  opposite  sides  of  the  head  with  the  diffi- 
culty of  analysis  in  Ex.  83  b. 

Stumpf,  II. ,  336,  363. 

101.  Judgments   of  the   Direction   of   Sounds.      These 
depend  in  general  on  the  relative  intensity  of  the  sounds 
reaching  the  two  ears,  but  there  is  pretty  good  reason  to 
believe  that  other  factors  co-operate  and  that  tolerably  cor- 
rect judgments,  both  as  to  distance  and  direction,  can  some- 
times be  made  from  the  sensations  of  one  ear. 

a.  Let  the  subject  be  seated  with  closed  eyes.     Snap  the 
telegraph  snapper  at  different  points  in  space  a  foot  or  two 
distant  from  his  head,  being  very  careful  not  to  betray  the 
place  in  any  way,  and  require  him  to  indicate  the  direc- 
tion of  the  sound.     Try  points  both  in  and  out  of  the  median 
plane.     Observe  that  the  subject  seldom  or  never  confuses 
right  and  left  but  often  makes  gross  errors  in  other  direc- 
tions.    Constant  tendencies  to  certain  locations  are  by  no 
means  uncommon. 

b.  Have  the  subject  hold  his  hands  against  the  sides  of 
his  head  like  another  pair  of  ears,  hollow  backward,  and 
try  the  effect  upon   his  judgment  of  the  direction  of  the 
snapper. 


SENSATIONS    OF  HEARING.  83 

c.  Find   approximately   how   far   the   snapper   must  be 
moved  vertically  from  the  following  points  in  order  to  make 
a  just  observable  change  in  location :  on  a  level  with  the  ears 
in  the  median  plane  two  feet  in  front ;   opposite  one  ear, 
same  distance ;  in  the  median  plane  behind  the  head,  same 
distance.     Find  the   just   observable   horizontal   displace- 
ments at  the  same  points.     A  convenient  way  of  measuring 
these  distances  is  to  clamp  a  yard-stick  to  a  retort-stand, 
bring  it  into  the  line  along  which  measurements  are  to  be 
made  and  hold  the  snapper  over  the  divisions  of  the  stick. 
Snap  once  at  the  point  of  departure,  then  at  a  point  a  little 
way  distant  in  the  direction  to  be  studied ;  again  at  the 
first  point,  so  that  the  subject  may  keep  it  in  mind,  and 
then  at  a  point  a  little  more  distant,  and  so  on  till  a  point 
is  finally  found  which  the  subject  recognizes  as  just  obser- 
vably different.     Eepeat,  alternating  snaps  at  the  point  of 
departure  with  those  at  a  greater  distance  than  that  just 
found,  decreasing  the  latter  till  a  point  is  found  where  the 
directions  can  be  no  longer  distinguished.     Make  a  number 
of  tests  each  way  and  take  their  average. 

d.  Continuous  simple  tones  are  very  difficult  to  locate. 
Place  a  tuning-fork  on  its  resonance  case  at  some  distance 
in  front  of  the  subject  (seated  with  closed  eyes),  another  at 
an  equal  distance  behind  him.     With  the  help  of  an  assis- 
tant strike  both  forks,  and  after  a  little  have  one  of  them 
stopped  and  the  mouth  of  its  resonance  box  covered.     Ee- 
quire   the   subject  to  say  which  has   been   stopped.     His 
errors  will  be  very  frequent.     Compare  with  this  his  ability 
to  distinguish  whether  a  speaker  is  before  or  behind  him. 

On  a,  Preyer,  B ;  von  Kries,  A ;  on  c,  Miinsterberg,  B ;  on  d,  Kay- 
leigh. 

102.  Intercranial  Location  of  Sounds,  a.  Sounds  origi- 
nating outside  the  head  are  not  located  in  the  head  when 
heard  with  one  ear.  Hold  a  loud-sounding  tuning-fork 


84          LABORATORY  COURSE  IN  PSYCHOLOGY. 

near  the  ear,  or  place  it  on  a  rubber  tube,  one  end  of  which 
is  inserted  in  the  opening  of  the  ear,  and  notice  that  the 
sound  when  strong  may  be  located  in  the  ear,  but  does  not 
penetrate  farther.  Insert  the  other  end  of  the  tube  in  the 
opening  of  the  other  ear  and  repeat.  The  tone,  if  loud,  will 
appear  to  come  from  the  inside  of  the  head.  Removing  and 
replacing  the  fork  several  times  will  help  to  give  definite- 
ness  to  the  location. 

b.  Repeat  the  experiment,  but  use  a  fork  sounding  as 
faintly  as  possible  (e.g.,  set  in  vibration  by  blowing  smartly 
against  it),  and  notice  that  the  location,  when  a  single  ear 
receives  the  sound,  is  not  so  clearly  in  the  ear,  and,  when 
both  receive  it,  not  so  clearly  in  the  head,  perhaps  even 
outside  of  it.  Cf.  also  Ex.  103  b.  Both  a  and  b  may  also 
be  made  with  beating  tones  instead  of  a  single  one.  See 
also  Ex.  69  e. 

Schaefer,  B. 

103.  Location  of  the  Tones  of  Tuning-forks  Pressed 
against  the  Head.  a.  Strike  a  large  and  loud-sounding 
tuning-fork,  and  press  its  stem  against  the  vertex.  The 
tone  will  seem  to  come  from  the  interior  of  the  head,  chiefly 
from  the  back.  While  the  fork  is  in  the  same  position, 
close  one  of  the  ears  with  the  finger,  not  pressing  it  too 
tight;  the  sound  will  immediately  seem  to  concentrate  in 
the  closed  ear.  Have  an  assistant  manage  the  fork,  and 
close  the  ears  alternately.  Something  of  the  same  kind 
happens  when  a  deep  note  is  sung ;  close  first  one  ear  and 
then  both,  and  notice  the  passage  of  the  tone  from  the 
throat  to  the  ear  and  finally  to  the  middle  of  the  head. 

b.  Have  an  assistant  manage  the  fork,  and  close  both  ears. 
Notice  that  when  the  fork  is  pressed  on  so  as  to  make  the 
tone  loud  the  intercranial  location  is  exact,  but  when  the 
pressure  is  relaxed  and  the  tone  is  faint  the  location  tends 
to  be  extracranial. 


SENSATIONS    OF  HEARING.  85 

c.  Try  setting  the  fork  on  other  places  than  the  vertex. 
Notice  that  in  the  occipital  and  parietal  regions  the  sound 
appears  in  the  opposite  ear,  though  closing  the  ear  as  in 
a  may  bring  it  back  to  the  same  side  as  the  fork. 

d.  Take  a  long  pencil  in  the  teeth  like  a  bit  and  rest  the 
stem  of   a  vibrating  tuning-fork  vertically  on  it  near  one 
end  and  close  the  ear  on  the  other  side ;  the  sound  will 
seem  to  be  located  in  the  closed  ear.     Then  gradually  tilt 
the  fork  backward  toward  a  horizontal  position,  keeping  it 
in  contact  with  the  pencil,  till  its  tip  is  opposite  the  open 
ear.     The  tone  will  change  its  place  from  the  closed  to  the 
open  ear. 

On  a  and  6,  Schaefer,  B  and  C ;  on  c,  Thompson. 


BIBLIOGKAPHY. 

BRUCKE:  Ueber  die  Wahrnehmung  der  Gerausche,  Wien.,  Sitzb.  3te 

Abth.,  XC.,  1884,  199-230. 
VON  BEZOLD:  A.  Schuluntersuchungen  iiber  das  kindliche  Gehoror- 

gan,  Zcitsch.f.  Ohrenheilkynde,  XIV.,  1884-85,  and  XV.,  1885- 

86;  also  in  English  translation  in  the  Archives  of  Otology,  XIV. 

This  paper  gives    the  results  of  numerous   tests  on  Munich 

school-children,  not  only  with  the  watch  but  also  with  the  acou- 

meter  of  Politzer  and  with  whispered  speech. 
J5.  Einige  weitere  Mitteilungen  iiber  die  kontinuierliche  Tonreihe, 

insbesondere  iiber  die  physiologische  obere  und  untere  Ton- 

grenze,  ibid.,  XXIII.,  1892,  254-267;  also  in  English  translation, 

Archives  of  Otology,  1893,  216-225. 
CORRADI:   Zur  Priifung  der  Schallperception   durch  die  Knochen, 

Archiv  fur  Ohrenheilkunde,   XXX.,   1890,   175-182.      Review 

with  extract  in  the  Zeitschrift  fiir  Psychologic,  II.,  1891,  124. 
CHARPENTIER:    Recherches  sur  1'intensite   comparative   des   sons 

d'apres  leur  tonalite,  Archives  de  physiologic,  normale  etpatho- 

logique,  1890,  No.  3,  496-507. 


86  LABORATORY   COURSE  IN  PSYCHOLOGY. 

CROSS  AND  GOODWIN:  Some  Considerations  regarding  Helniholtz's 
Theory  of  Consonance,  Proceedings  of  the  American  Academy 
of  Arts  and  Sciences,  1891-92,  1-12. 

CROSS  AND  MALTBY:  On  the  Least  Number  of  Vibrations  Neces- 
sary to  Determine  Pitch,  ibid.,  222-235. 

DOCQ:   Kecherches  physico-physiologique  sur  la  fonction  collective 
des  deux  organs  de  1'appareil  auditif.     Memoir es  couronnes  de 
I' Academic  royale  de  Belgique,  XX5QV.,  1870. 

EXNER:  Zur  Lehre  von  den  Gehorsempfindungen,  Pfluger's  Archiv, 
XIII. ,  1876,  228-253. 

HELMHOLTZ  :  Sensations  of  Tone,  English  translation  by  Ellis,  2d 
Ed.,  London,  1885.  This  is  the  great  classic  of  the  subject. 

HENSEN:  Physiologic  des  Gehors,  Hermann's  Handbuch  der  Physio- 
logic, III.,  pt.  2,  1-137. 

HERMANN:  Zur  Theorie  der  Combinationstone,  Pjluger's  Archiv, 
XLIX.,  1891,  499-518. 

HERROUN  AND  YEO:  Note  on  the  Audibility  of  single  Sound 
Waves  and  the  Number  of  Vibrations  necessary  to  produce  a 
Tone,  Proc.  Royal  Soc.,  L.,  No.  305,  1892,  318-323. 

JAMES  :  Principles  of  Psychology,  New  York,  1890. 

KESSEL  :  Ueber  die  vordere  Tenotomie,  Archiv  fur  Ohrenheilkunde, 
XXXI.,  1891,  131-143,  Keviewed,  Zeitschrift  fur  Psychologic, 
II.,  1891,  398. 

KONIG:   Quelques  experiences  d'acoustique,  Paris,  1882. 

VON  KRIES:  A.  Ueber  das  Erkennen  der  Schallrichtung,  Zeitschrift 

fur  Psychologie,  I.,  1890,  235-251,  488. 
B.  Ueber  das  absolute  Gehor,  Ibid.,  III.,  1892,  257-279. 

LANGE  :  Beitrage  zur  Theorie  der  sinnlichen  Aufmerksamkeit  und 
der  activen  Apperception,  Wundfs  Philosophiscfie  Studien, 
IV.,  1888,  390-422. 

LORENZ  :  Untersuchungen  iiber  die  Auffassung  von  Tondistanzen, 
WundV  s  Philos.  Studien,  VI.,  1890,  26-103. 

LUFT  :  Ueber  die  Unterschiedsempfindlichkeit  fur  Tonhohen. 
Wundfs  Philos.  Studien,  IV.,  1888,  511-540. 

MACH:  Works  cited  with  same  letters  in  bibliography  of  Chap.  II. 


SENSATIONS   OF  HEARING.  87 

MAYEB:  A.  Kesearches  in  Acoustics,  Amer.  Jour.  Science,  3d  Ser. 

VIII.,  1874,  241-255,  IX.,  1875,  267-269,  also  Phil.  Mag.,  4th 

Ser.  XLIX.,  Jan.-June,  1875,  352. 
B.   Kesearches  in  Acoustics,  No.  VIII.,  Amer.  Jour.  Sc.,  3d  Ser. 

XII!,  1876,  329-336,  also  Phil.  Mag.,  Ser.  5,  II. ,  July-Dec., 

1876,  500-507. 
MUNSTERBERG:   A.  Schwanktmgen  der  Aufmerksamkeit,  Beitrdge 

zur  experimentellen  Psychologic,  Heft  2,  1889,  69-124. 

B.  Raumsinn  des  Ohres,  Ibid.,  182-234. 

C.  Vergleichung  von  Tondistanzen,  Ibid.,  Heft  4,  1892,  147-177. 
PREYER:  A.  Ueber  die  Grenzen  der  Tonwahrnehmung,  Sammlung 

physiologischer  Abhandlungen,  I.,  Jena,  1877,  1-72. 

B.  Die  Wahrnehmung  der   Schallrichtung  mittelst  der  Bogen- 
gange,  Pfluger's  Archiv,  XL.,  1887,  586-622. 

C.  Ueber  Combinationstone,    Wiedemann's  Annalen,  XXXVIII., 
1889,  131-136. 

D.  Akustische  Untersuchungen,  Sammlung  physiologischer  Ab- 
handlungen, II.,  Jena,  1882,  175-244. 

RAYLEIGH:  Our  Perception  of  the  Direction  of  a  Source  of  Sound, 
Nature,  XIV.,  1876,  32.  See  also  Acoustical  Observations, 
Phil.  Mag.,  Ser.  5,  III.,  Jan.-June,  1877,  456-458. 

RUTHERFORD  :  A  Lecture  on  the  Sense  of  Hearing,  delivered  before 
the  British  Association  at  Birmingham  on  Sept.  6,  1886, 
Lancet,  1887,  i.  2-6. 

SCHAEFER:  A.  Ueber  die  Wahrnehmung  und  Lokalisation  von 
Schwebungen  und  Differenztonen,  Zeitschrift  fur  Psychologie, 
I.,  1890,  81-98. 

B.  Zur  interaurealen    Lokalisation  diotischer  Wahrnehmungen, 
Ibid.,  I.,  1890,  300-309. 

C.  Ein  Versuch  iiber  die  intrakranielle  Leitung  leisester  Tone  von 
Ohr  zu  Ohr,  Ibid.,  II.,  1891,  111-114.     See  also  discussion  of 
Schaefer,  Scripture  and  Wundt,  Ibid.,  IV.,  348;  V.,  397;  and 
WundV s  Philos.  Studien,  VII.,  630;  VIII.,  638,  641. 

SCHISCHMANOW:  Untersuchungen  iiber  die  Empfindlichkeit  des  In- 
tervallsinnes,  Wundt1  s  Philos.  Studien,  V.,  1889,  558-600. 

STUMPF:  Tonpsychologie,  Leipzig,  1883  and  1890.  This  work  of 
Stumpf  s  is  by  far  the  most  complete  upon  the  Psychology  of 
Tone.  The  two  volumes  so  far  published  (the  work  is  to  be 
complete  in  four)  cover  the  psychology  of  successive  and  of 
simultaneous  tones. 


88  LABORATORY  COURSE  IN  PSYCHOLOGY. 

THOMPSON,  SYLVANUS  P.:  A.    On  Binaural  Audition,  Phil.  Mag., 

Ser.  5,  IV.,  July-Dec.,   1877,  274-276;  VI.,  July-Dec.,  1878, 

383-391;  XII. ,  July-Dec.,  1881,  351-355. 
B.  On  the  Function  of  the  Two  Ears  in  the  Perception  of  Space, 

Ibid.,  XIII.,  Jan.- June,  1882,  406-416. 
UKBANTSCHITSCH:  A.  Ueber  eine  Eigentiimlichkeit  der  Schallem- 

pfindungen  geringster  Intensitat,  Centralblatt  f.  d.  med.  Wis- 

sens.,  1875,  625-628. 

B.  Zur  Lehre  von  der  Schallempfindung,  Pflilger's  Archiv,  XXIV., 
1881,  574-595. 

C.  Ueber  das  An-    und    Abklingen    acustischer   Empfindungen, 
Ibid.,  XXV.,  1881,  323-342. 

WUNDT:  Work  cited  in  bibliography  of  Chap.  I.,  3teAufl.,  I.,  415 
ff.,  II.,  42  ff.;  4te  Aufl.,  I.,  443  ff. 

On  the  physics  and  physiology  of  sound,  reference  may  be  made,  in 
addition  to  the  works  already  mentioned,  to  Tyndall,  On 
Sound;  Blaserna,  Theory  of  Sound  in  its  Relations  to  Music; 
Zahn,  Sound  and  Music;  and  Taylor,  Sound  and  Music.  The 
last  is  very  simple  and  untechnical,  and  is  perhaps  the  best  for 
those  approaching  the  subject  for  the  first  time. 

For  the  Stumpf-Wundt  discussion  on  pitch  distances  consult  the 
following:  Stumpf,  Zeitschrift  fur  Psychologic,  I.,  1890,  419; 
II.,  1891,  266,426,  438;  Engel,  Ibid,  II.,  1891,  361;  Wundt, 
Philos.  Studien,  VI.,  1890-91,  605;  VII.,  1891,  298,  633;  also 
Miinsterberg,  C,  above. 


THE  MECHANISM  OF  THE  EYE.  89 


CHAPTER   V. 
The  Mechanism  of  the  Eye  and  Vision  in  General. 

THE  mechanism  of  the  eye  accomplishes  two  things  :  the 
projection  of  a  sharp  image  on  the  retina,  and  the  ready 
shifting  of  the  eye  so  as  to  bring  successive  portions  of  the 
image  into  the  best  position  for  seeing.  To  the  study  of 
these  mechanisms  and  other  physiological  phenomena  of 
importance  for  the  psychology  of  vision,  this  chapter  is 
devoted. 

THE  KETINAL  IMAGE  AND  ACCOMMODATION. 

104.  The  Eetinal  Image.  This  is  easily  seen  in  the 
unpigmented  eye  of  a  pink-eyed  rabbit. 

a.  Chloroform  the  rabbit,  remove  the  eyes,  and  mount 
them  in  clay  for  readier  handling.  The  mounting  is  done 
as  follows  :  Make  a  thick  ring  of  clay  with  an  internal 
diameter  a  little  greater  than  that  of  the  cornea  of  the 
rabbit's  eye ;  place  the  eye,  cornea  downward,  in  the  ring ; 
lay  a  similar  ring  upon  it  to  keep  it  in  place,  and  press  the 
edges  of  the  rings  together.  The  eye  can  now  be  handled 
easily  and  turned  in  any  direction.  Turn  the  cornea 
toward  the  window,  and  observe,  from  behind,  the  inverted 
image  on  the  retina.  Bring  the  hand  into  range  and  move 
it  to  and  fro ;  observe  that  the  image  of  distant  objects  is 
more  distinct  than  that  of  the  hand.  The  dead  eye  is 
adjusted  for  distant  vision.  If  convex  and  concave  lenses 
are  at  hand  (spectacle  lenses  will  answer),  bring  them 
before  the  eye,  and  observe  that  the  effect  upon  the 


90  LABORATORY   COURSE  IN  PSYCHOLOGY. 

retinal  image  is  similar  to  that  seen  subjectively  when 
they  are  held  before  the  observer's  own  eye,  provided 
that  that  is  normal. 

Reverse  the  eye,  holding  it  retina  side  toward  the  win- 
dow, and  observe  the  radiating  and  circular  fibres  of  the 
iris.  The  eye  must  be  fresh,  for  if  long  removed  it  loses 
its  transparency. 

105.  Accommodation.     The    sharpness    of    the    retinal 
image  depends  on  the  adjustment  of  the  crystalline  lens, 
which  must  be  such  as  to  focus  upon  the  retina  the  light 
from  the  object  under  regard.     The  lens  must  be  thicker 
and  rounder  for  near  objects,  thinner  and  flatter  for  more 
distant  ones.     These  adaptations  of  the  eye  are  known  as 
Accommodation.     The  changes  in  the  clearness  of  the  retinal 
image  are  easy  to  observe  subjectively.     Hold  up  a  pin  or 
other  small  object  six  or  eight  inches  away  from  the  eyes. 
Close  one  eye  and  look  at  the  pin  with  the  other.     The  out- 
line of  the  pin  is  sharp,  but  the  outlines  of  things  on  the 
other  side  of  the  room  behind  it  are  blurred.     Look  at  these, 
and  the  outline  of  the  pin  becomes  blurred.     Notice  the 
feeling  of  greater  strain  when  looking  at  the  nearer  object. 
The  experiment  is  somewhat  more  striking  when  the  nearer 
object  is  a  piece  of  veiling  or  wire  gauze,  and  the  farther,  a 
printed  page  held  at  such  a  distance  that  it  can  just  be 
read. 

On  this  and  the  next  two  experiments,  see  Helmholtz,  A,  112-118, 
Fr.  119-126  (90-96). 

106.  Schemer's  Experiment,     a.  Pierce  a  card  with  two 
fine  holes  separated  by  a  less  distance  than  the  diameter  of 
the  pupil,  say,  a  sixteenth  of  an  inch.     Set  up  two  pins  in 
corks,  distant  respectively  eight  and  twenty  inches  from 
the  eye  in  the  line  of  sight ;  close  one  eye,  and  holding  the 
card  close  before  the  other  with  the  holes  in  the  same  hori- 


THE  MECHANISM  OF   THE  EYE. 


91 


zontal  line,  look  at  the  nearer  pin;  the  farther  pin  will 
appear  double.  Look  again  at  the  nearer  pin,  and  while 
looking,  cover  one  of  the  holes  with  another  card ;  one  of 
the  images  of  the  farther  pin  will  disappear  —  the  left  when 
the  left  hole  is  covered,  and  the  right  when  the  right  is 
covered.  Look  at  the  farther  pin  or  beyond  it ;  the  nearer 
pin  appears  double.  Repeat  the  covering ;  closing  the  left 
hole  now  destroys  the  right  image,  and  covering  the  right 
destroys  the  left. 


Why  this  should  be  so  will  be  clear  from  the  diagrams 
above.  The  upper  diagram  illustrates  the  course  of  the  rays 
of  light  when  the  eye  is  accommodated  for  the  nearer  pin ; 
the  lower  diagram  when  it  is  accommodated  for  the  farther 
pin.  A  and  B  represent  the  pins;  S  and  S  the  pierced 
screen ;  d  and  df  the  holes  in  the  screen ;  c  and  c  the  lens ; 
a'  b  a"  and  V  a  V  the  retinae ;  A,  A',  Br  and  B" ',  the  positions 
of  the  double  images.  The  solid  lines  represent  the  course  of 
the  rays  from  the  pin  that  is  accommodated  for ;  the  lines 
of  short  dashes,  the  course  of  the  rays  from  the  other  pin ; 


92  LABORATORY  COURSE  IN  PSYCHOLOGY. 

the  lines  of  long  dashes,  the  lines  of  direction ;  i.e.,  approxi- 
mately those  giving  the  direction  in  which  the  images 
appear  to  the  observer.  In  the  upper  diagram  the  rays 
from  B  are  focused  to  a  single  retinal  image  at  b,  while 
those  from  A,  being  less  divergent  at  first,  are  brought  to 
a  focus  nearer  the  lens,  cross  over  and  meet  the  retina  at 
a!  and  a",  and,  since  each  hole  in  the  screen  suffices  to 
produce  an  image,  cause  the  pin  to  appear  double.  Its  two 
images  are  referred  outward  as  all  retinal  images  are,  along 
the  lines  of  direction  (which  cross  a  little  forward  of  the 
back  surface  of  the  lens,  in  the  crossing  point  of  the  lines  of 
direction),  the  right  retinal  image  corresponding  with  the 
left  of  the  double  images  and  vice  versa.  If  now  the  right 
hole  (d)  in  the  screen  be  closed,  the  left  retinal  image  and 
the  right  double  image  disappear.  The  case  of  accommo- 
dation for  the  farther  pin  will  be  clear  from  the  lower 
diagram,  if  attention  is  given  to  the  dotted  and  dashed 
lines.  It  will  also  be  easy  to  explain  why  moving  the  card 
when  looking  through  a  single  pin-hole  causes  apparent 
movements  of  the  pin  not  accommodated  for,  and  why  in 
one  case  the  movement  seems  to  be  with  the  card,  and  in  the 
other  case  against  it. 

b.  Stick  the  pins  into  the  corks  so  that  they  shall  extend 
horizontally,  and  examine  them  with  the  card  held  so  as  to 
bring  the  holes  one  above  the  other. 

c.  Arrange   the   holes   thus :  .  • .  and   observe   that  the 
triple  image  of  the  nearer  pin  (when  the  farther  is  fixated) 
has  the  reverse  figure 

Schemer's  experiment  can  easily  be  illustrated  with  any 
convex  lens  and  a  pierced  screen  of  suitable  size. 

107.  Range  of  Accommodation,  a.  Find  by  trial  the 
nearest  point  at  which  a  pin  seen  as  in  Schemer's  experi- 
ment can  be  seen  single.  This  is  the  near  point  of  accom- 


THE  MECHANISM  OF   THE  EYE.  93 

modation.     For  the  short-sighted  a  far  point  may  also  be 
found,  beyond  which  double  images  reappear. 

b.  Find  how  far  apart  in  the  line  of  sight  two  pins  may 
be,  and  yet  both  be  seen  single  at  one  and  the  same  time. 
Try  with  the  nearer  at  20  cm.,  at  50  cm.,  at  2  m.  That 
portion  of  the  line  of  sight,  for  points  in  which  the  same 
degree  of  accommodation  is  sufficient,  is  called  the  Line  of 
Accommodation.  The  length  of  the  line  increases  rapidly  as 
the  distance  of  the  object  from  the  eye  increases. 

Helmholtz,  A,  114,  119,  Fr.  122  (93),  128  (97). 

108.  Mechanism  of  Accommodation.  The  change  in  the 
lens  in  accommodation  is  chiefly  a  bulging  forward  of  its 
anterior  surface.  This  may  be  observed  as  follows  :  — 

a.  Let  the  subject  choose  a  far  and  a  near  point  of  fixation 
in  exactly  the  same  line  of  vision  ;  close  one  eye  and  fix  the 
other  upon  the  far  point.     Let  the  observer  place  himself 
so  that  he  sees  the  eye  of  the  subject  in  profile  with  about 
half  the  pupil  showing.     Let  the  subject  change  his  fixation 
at  request,  from  the  far  to  the  near  point,  and  vice  versa, 
being  careful  to  avoid  any  sidewise  motion   of  the  eye. 
The  observer  will  notice,  when  the  eye  is  accommodated  for 
the  near  point,  that  more  of  the  pupil  shows  and  that  the 
farther  side  of  the  iris  seems  narrower.     This  change  is 
due  to  the  bulging  forward  of  the  front  of  the  lens.     If 
the   change   were   due   to   accidental   turning  of   the   eye 
toward  the  observer,  the  farther  edge  of  the  iris   should 
appear  wider  instead  of  narrower.     Notice  also  that  the 
diameter  of  the  pupil  changes  with  the  accommodation. 

b.  Purkinje's  Images.     The  changes  in  the  curvature  of 
the  lens  may  also  be  observed  by  means  of  the  images 
reflected  from  its  front  and  back  surfaces  and  from  the  front 
of  the  cornea.     Operate  in  a  darkened  room.     Let  the  sub- 
ject choose  far  and  near  fixation  points  as^beJor^Let  the 


94  LABORATORY  COURSE  IN  PSYCHOLOGY. 

observer  bring  a  candle  near  the  eye  of  the  subject  at  a 
level  with  it  and  a  little  to  one  sideband  place  his  own  eye 
in  a  position  symmetrical  to  the  candle  on  the  other  side  of 
the  subject's  line  of  sight.  Careful  examination  and  some 
shifting  about  of  the  place  of  the  candle  and  of  the  observer 
will  show  three  reflected  images  of  the  flame :  one  on  the 
side  of  the  pupil  next  the  light,  easily  recognizable,  bright 
and  erect,  reflected  from  the  surface  of  the  cornea ;  a  second, 
nearer  the  centre  of  the  pupil  and  apparently  the  farthest 
back  of  the  three,  erect  like  the  first,  but  very  indistinct 
(more  like  a  light  cloud  than  an  image),  reflected  from  the 
anterior  surface  of  the  lens ;  and  a  third,  a  mere  point  of 
light,  near  the  side  of  the  pupil  farthest  from  the  flame, 
inverted  and  reflected  from  the  posterior  surface  of  the 
lens.  When  the  observer  has  found  these  three  images,  the 
subject  should  fixate  alternately  the  near  and  far  points 
chosen.  As  he  fixates  the  near  point,  the  middle  image 
will  grow  smaller,  advance,  and  draw  toward  the  corneal 
image ;  when  he  fixates  the  far  point,  the  image  will  enlarge, 
recede,  and  move  away  from  the  corneal  image.  The  follow- 
ing diagram,  after  Aubert,  illustrates  the  movement  of  the 

middle  image ;  the  full 
lines  indicate  the  posi- 
tions of  the  cornea  and 
lens  and  the  course  of 
the  rays  of  light  when 
the  eye  is  accommodated 
for  the  far  point ;  the 
dotted  lines  indicate 
the  anterior  surface  of 
the  lens  and  the  direc- 
tion of  the  ray  reflected 
from  its  surface  when  the  eye  is  accommodated  for  the  near 
point.  Three  images  similar  to  those  in  question  can  be 


THE  MECHANISM  OF   THE  EYE.  95 

observed  on  a  watch  glass  and  a  double  convex  lens  held  in 
the  relation  of  the  cornea  and  crystalline.1 

Helmholtz,  A,  131-141,  especially  131-134,  Fr.  142-154  (104-112), 
especially  ,142-146  (104-107);  Aubert,  A,  444;  Tscherning. 

109.  Dioptrical  Defects  of  the  Eye.  Of  these  defects 
only  two  will  be  considered  here :  Astigmatism  and  Chro- 
matic Aberration.  The  first  is  an  error  in  the  form  or  set- 
ting of  the  refracting  surfaces,  which  prevents  their  bringing 
parallel  light  to  a  focus  in  a  single  point.  If  the  curvature 
of  the  lens,  for  example,  (or  of  the  cornea),  is  greater  on  the 
vertical  meridian  than  on  the  horizontal,  parallel  light  fall- 
ing upon  the  first  will  be  brought  to  a  focus  nearer  the  lens 
than  that  falling  upon  the  second.  This  makes  it  impossible 
for  the  astigmatic  eye  to  see  all  parts  of  a  plane  figure  with 
equal  distinctness  at  the  same  time.  Chromatic  Aberration 
depends  upon  the  different  degrees  of  refraction  which  dif- 
ferent colored  lights  experience  in  traversing  the  lens ; 
those  of  short  wave-length  (violet  and  blue)  are  most  re- 
fracted, those  of  long  wave-length  (red  and  orange)  least, 
and  the  others  in  order  between.  The  point  at  which  paral- 
lel violet  rays  are  brought  to  a  focus  is  therefore  nearer  the 
lens  than  the  point  for  red.  In  order,  therefore,  that  the 
same  degree  of  accommodation  may  serve  to  show  a  red 
lighted  object  and  a  violet  lighted  object  at  the  same  time 
and  both  with  full  distinctness,  the  red  light  must  be  less 
divergent  than  the  violet ;  in  other  words,  the  red  lighted 
object  must  be  somewhat  farther  away. 

a.  Astigmatism.  Make  a  fine  pin-hole  in  a  card ;  hold  it 
at  arm's  length  against  a  bright  background  and  accommo- 

1  By  using  a  magnifying-glass  a  second  faint  corneal  image  very  close  to  the 
first  can  be  seen,  when  the  light  strikes  the  cornea  well  toward  one  side.  When 
this  is  counted,  as  it  is  by  Tscherning,  there  are  four  Purkinje  images,  those  from 
the  front  and  back  of  the  lens  becoming  the  third  and  fourth  in  the  enumeration, 
instead  of  the  second  and  third. 


96 


LABORATORY  COURSE  IN  PSYCHOLOGY. 


date  the  eye  for  a  nearer  point,  or  put  on  convex  glasses. 
The  spot  will  not  appear  as  a  little  circle  of  light,  as  it 
would  if  the  lens  and  cornea  were  perfect  in  form,  but  as  a 
more  or  less  irregular  star  or  flower-shaped  figure  in  which 
portions  of  several  images  of  the  hole  may  be  made  out. 
Accommodate  for  a  point  considerably  beyond  the  card 
and  notice  the  change  in  the  figure. 

These  irregularities  (phenomena  of  Irregular  Astigma- 
tism) disappear,  however,  with  exact  accommodation,  but 
another  kind  (Regular  Astigmatism)  is  then  to  be  observed. 
Close  one  eye  and  look  with  the  other  at  the  centre  of  the 
radiating  figure  below.  Notice  which  lines  appear  with 
greatest  blackness  and  distinctness.  Try  the  effect  of 
increasing  and  decreasing  the  distance.  Try  also  the  other 
eye. 


Something  of  the  same  kind  is  to  be  seen  in  the  set  of 
concentric  circles ;  also  evidences  of  irregular  astigmatism 
when  accommodation  is  changed  or  when  the  distance  of 
the  diagram  is  increased  or  decreased.  Notice  especially 
the  rayed  appearance  and  the  distortion  of  the  inner  circles 
when  the  eye  is  accommodated  for  a  greater  distance  than 


THE  MECHANISM  OF  THE  EYE.  97 

that  of  the  diagram.  On  the  latter  peculiarity,  see  von 
Bezold. 

b.  Chromatic  Aberration.  Bend  a  fine  platinum  wire 
into  a  ring  half  an  inch  in  diameter,  and  heat  it  white  hot 
in  the  flame  of  a  Bunsen  burner.  Look  at  the  ring  through 
a  pin-hole  in  a  black  card  held  at  such  a  distance  that  the 
ring  lies  close  to  the  edge  of  the  field  of  the  pin-hole  all 
around.  Accommodate  the  eye  for  the  centre  of  the  ring, 
and  observe  that  the  outer  edge  of  the  ring  appears  bright 
red,  the  inner  edge  blue  or  violet.  Substitute  for  the  card 
.a  bit  of  blue  glass,  and  accommodate  first  for  the  glass,  then 
for  a  point  some  distance  beyond  the  ring.  In  the  first 
case  the  outer  and  inner  edges  of  the  ring  (except  as  astig- 
matism interferes)  will  both  be  blue;  in  the  second  case 
they  will  be  red.  The  ordinary  blue  glass  allows  both  red 
and  blue  light  to  pass  through  it. 

Look  at  the  edge  of  the  window  frame  next  the  pane, 
and  bring  a  card  before  the  eye  so  that  about  half  the 
pupil  is  covered  ;  if  the  card  has  been  brought  up  from  the 
frame  side,  the  frame  will  be  bordered  with  yellow ;  if 
from  the  pane  side,  with  blue.  In  ordinary  vision  these 
fringes  do  not  appear,  because  the  colors  partially  overlap 
and  produce  a  practically  colorless  mixture. 

Yon  Bezold's  Experiment.  Look  at  the  parallel  lines  of 
the  left  figure  in  Ex.  118  with  imperfect  accommodation, 
e.g.,  through  convex  spectacles,  and  observe  the  aberra- 
tion colors.  If  a  set  of  heavy  concentric  circles  (separated 
by  equal  spaces,  and  beginning  with  a  central  black  dot 
of  a  diameter  equal  to  the  width  of  the  lines)  is  used 
instead  of  the  straight  line  figure,  it  will  be  possible  by 
changing  its  distance  from  the  eye  to  find  a  position  in 
which  the  aberration  colors  so  overlap  that  dark  and  light 
seem  to  have  changed  places,  and  the  central  spot  is  light 
instead  of  dark.  The  spiral  figure  with  Ex.  128  will  show 


98  LABORATORY  COURSE  IN  PSYCHOLOGY. 

something  of  the  effect,  but  the  central  black  spot  is  too 
large  to  show  it  completely. 

Both  astigmatic  differences  and  the  aberration  colors  may 
at  times  influence  judgments  of  distance. 

On  a,  Helmholtz,  A,  169  ff.,  Fr.  187  (138)  ft.  On  6,  Helmholtz 
A,  156-164,  Fr.  172-179  (125-131);  von  Bezold;  Tumlirz. 

ENTOPTIC  APPEARANCES. 

110.  Floating  Particles  in  the  Media  of  the  Eye  and  on 
its  Surface;  Muscce  Volitantes.  Fix  a  lens  of  short  focus 
at  some  distance  from  a  bright  gas  or  candle  flame.  Set  up- 
in  the  focus  of  the  lens  a  card  pierced  with  a  very  fine  hole ; 
bring  the  eye  close  to  the  hole  and  look  toward  the  light. 
The  eye  should  be  far  enough  from  the  hole  to  prevent  the 
edge  of  the  lens  from  being  seen.  The  rays  of  light  that 
now  reach  the  eye  are  strongly  divergent,  and  the  crystalline 
lens  does  not  bring  them  to  a  focus  on  the  retina,  but  only 
refracts  them  to  such  a  degree  that  they  traverse  the  eye 
nearly  parallel,  and  thus  in  suitable  condition  for  casting 
sharp  shadows  upon  the  retina  of  objects  on  or  in  the  eye. 

a.  The  lens  will  appear  full  of  light,  and  in  it  will  be 
seen  a  variety  of  shadings,  blotches,  and  specks,  single  or  in 
strings,  the  outward  projection  of  the  shadows  just  men- 
tioned. The  figures  in  this  luminous  field  will  vary  from 
person  to  person,  even  from  eye  to  eye,  but  in  almost  every 
eye  some  will  be  found  that  move  and  some  that  remain 
fixed  or  only  move  with  the  eye.  Of  the  moving  figures 
some  are  due  to  particles  and  viscous  fluids  on  the  surface 
of  the  eye  ;  they  seem  to  move  downward,  and  &re  changed 
by  winking.  Notice,  for  example,  the  horizontal  bands  that 
follow  a  slow  dropping  and  raising  of  the  upper  lid.  Such 
appearances  as  these,  since  their  cause  is  not  really  in  the 
eye  but  outside  of  it,  have  been  called  pseudentoptic  by 
Laqueur.  Others,  the  muscce  volitantes,  are  frequently 


THE  MECHANISM  OF  THE  EYE.  99 

noticed  without  any  apparatus  ;  they  appear  as  bright 
irregular  threads,  strings^  of  beads,  groups  of  points,  or 
single  minute  circles  with  light  centres.  They  seem  to 
move  downward  in  the  field,  but  actually  move  upward  in 
the  vitreous  humor  where  they  are  found.  Of  the  per- 
manent figures,  some  are  due  to  irregularities  of  structure 
or  small  bodies  in  the  crystalline  and  its  capsule  (spots 
with  dark  or  bright  centres,  bright  irregular  lines,  or 
dark  radiating  lines  corresponding  probably  to  the  radial 
structure  of  the  lens) ;  others  of  a  relatively  permanent 
character,  it  is  said,  can  be  produced  on  the  cornea  by 
continued  rubbing  or  pressure  on  the  eyeball. 

b.  The  rouncj  spot  of  light  in  which  these  things  are  seen 
represents  the  pupil,  and  the  dark  ground  around  it  is  the 
shadow  of  the  iris.  Notice  the  change  in  the  size  of  the 
spot  of  light,  as  the  eye  is  accommodated  for  different  dis- 
tances (cf.  Ex.  108),  or  as  the  other  eye  is  exposed  to,  or 
covered  from,  the  light.  The  change  begins  in  about  half 
a  second.  It  shows  the  close  connection  of  the  iris 
mechanisms  of  the  two  eyes,  and  is  typical  of  the  way  in 
which  the  two  eyes  co-operate  as  parts  of  a  single  visual 
organ. 

Some  of  these  entoptic  observations  may  be  made  with  a 
pierced  card  alone,  or  simply  by  looking  directly  at  a  broad 
expanse  of  clear  sky  without  any  apparatus  at  all. 

Helmholtz,  A,  184-192,  and  Tafel  I.,  which  shows  the  appearance 
of  several  of  these  entoptic  objects,  Fr.  204-214  (149-156)  and  PL  V., 
also  548-558  (419-427);  Laqueur. 

111.  Eetinal  Blood-vessels,  Purkinje's  Vessel  Figures. 
a.  Concentrate  a  strong  light  (preferably  in  a  dark  room), 
or  even  direct  sunlight,  with  a  double  convex  lens  of  short 
focus  on  the  sclerotic  in  the  outer  corner  of  the  eye  of  the 
subject,  requesting  him  to  turn  the  eye  toward  the  nose 


100         LABORATORY  COURSE  IN  PSYCHOLOGY. 

/ 

and  giving  him  a  dark  background  to  look  toward.  Mate 
the  spot  of  light  on  the  sclerotic  as  small  and  sharp  as 
possible,  and  give  to  the  lens  a  gentle  to  and  fro  or  circular 
motion.  After  a  little  the  subject  will  see  upon  the  field, 
which  the  light  makes  reddish-yellow,  the  dark  branching 
figure  of  the  shadows  of  the  retinal  vessels.  Notice  that 
the  spot  directly  looked  at  is  partially  surrounded,  but  not 
crossed,  by  the  vessels.  In  this  lies  the  yellow  spot  (macula 
luted) ,  the  retinal  area  of  clearest  vision.  The  centre-  from 
which  the  vessels  radiate  lies  in  the  point  of  entrance  of 
the  optic  nerve.  In  this  form  of  the  experiment  the  light 
radiates  in  all  directions  within  the  eye  from  the  illumi- 
nated point  of  the  sclerotic. 

b.  Somewhat  the  same  sort  of  image  is  to  be  secured  by 
moving  a  candle  about  near  the  eye,  below  it  and  a  little  to 
one  side.     In  this  experiment  some  indication  of  the  region 
of  the  yellow  spot  is  to  be  seen.     This  time  the  light  enters 
by  the  pupil,  forms  an  image  on  a  part  of  the  retina  some- 
what remote  from  the   centre,  and  this   retinal  image  is 
itself  the  source  of  the  light  by  which  the  vessel  shadows 
are  cast. 

c.  Look  through  a  pin-hole  in  a  card,  held  close  before 
the  eye,  at  the  sky  or  some  other  illuminated  surface,  or  at 
a  broad  gas-flame.     Give  the  card  a  rather  rapid  circular 
motion,  and  the  finer  retinal  vessels  in  the  region  of  the 
yellow  spot  will  readily  be  seen,  among  them  also  a  small 
colored  or  slightly  tinted  spot  (best  seen,  perhaps,  by  gas- 
light) representing  the  macula,  and  in  its  centre  a  shadowy 
dot  (representing  the  fovea,  the  point  of  clearest  vision), 
which  appears  to  rotate  when  the  motion  of  the  card  is 
circular.     If  the  card  is  moved  horizontally,  the  vertical 
vessels  alone  appear;  if  vertically,  the  horizontal  vessels. 
Notice  also  the  granular  appearance  of  the  macula;  the 
granulations  have  been  supposed  to  represent  the  visual 


THE  MECHANISM  OF  THE  EYE.  101 

cones  of  that  region.  The  finer  retinal  vessels  can  also  be 
seen  when  looking  at  the  vacant  field  of  a  compound  micro- 
scope, if  the  eye  is  moved  about  rapidly. 

In  ail  cases  it  is  important  that  the  shadows  be  kept 
moving  ;  if  they  stand  still,  they  are  lost.  The  explanation 
is  partly  physiological  (the  portions  of  the  retina  on  which 
the  shadows  rest  soon  gain  in  sensitiveness  enough  to  com- 
pensate for  the  less  light  received)  and  partly  psychological 
(moving  objects  in  general  arouse  spontaneous  attention,  and 
those  whose  images  rest  continuously  on  the  retina  without 
motion  are  particularly  subject  to  neglect). 

Once  having  become  familiar  with  these  vessel  figures,  it 
is  often  possible  for  the  observer  to  see  traces  of  them 
without  any  apparatus.  Parts  of  them,  with  something  of 
the  yellow  spot,  may  sometimes  be  seen  for  an  instant  as 
dark  figures  on  the  diffusely  lighted  walls  and  ceiling,  or  as 
light  figures  on  the  dark  field  of  the  closed  eyes,  when  the 
eyes  are  opened  and  closed  after  a  glance  at  the  window  on 
first  waking  in  the  morning,  or  as  blue  figures  when  looking 
at  the  snow  and  winking  on  a  bright  winter  morning. 

Helmholtz,  A,  192-198,  555,Fr.  214-221  (156-161),  528  (402). 

112.  Eetinal  Circulation.  Look  steadily  through  two  or 
three  thicknesses  of  blue  glass  at  the  clear  sky  or  a  bright 
cloud,  and  observe  the  bright  points  darting  hither  and 
thither  like  bees  in  a  swarm  or  snowflakes  on  a  windy  day. 
Careful  observation  will  also  establish  that  the  bright  points 
are  followed  by  shadowy  darker  ones.  Pick  out  a  speck  on 
the  window  to  steady  the  eyes,  and  observe  that  while  the 
movements  of  the  points  seem  irregular  the  same  lines  are 
retraced  by  them  from  time  to  time.  When  several  of 
their  courses  have  been  accurately  determined  for  one  of 
the  eyes,  repeat  the  experiment  for  demonstrating  the  finer 
retinal  vessels  (Ex.  Ill,  c),  and  notice  that  fine  vessels 


102       LABORATORY  COURSE  IN  PSYCHOLOGY. 

are  found  which  correspond  to  the  courses  that  the  points 
seem  to  follow.  These  flying  points  can  be  seen  without 
the  glass  by  a  steady  gaze  at  an  evenly  lighted  bright  sur- 
face, and  sometimes  a  rhythmical  acceleration  of  their 
movements  will  be  found,  corresponding  to  the  pulse. 
Helmholtz  explains  the  phenomenon  by  a  temporary  clog- 
ging of  fine  capillary  vessels  by  large  blood  corpuscles. 
The  bright  lines  (the  apparent  tracks  of  bright  points)  are 
really  the  relatively  empty  capillary  tubes  ahead  of  the  cor- 
puscles, which,  after  an  instant,  are  driven  onward  by  others 
crowding  behind,  which  in  turn  give  the  shadow  that  ap- 
parently follows  the  bright  points. 

Helmholtz,  A,  198  f.,  Fr.  221  (837),  555  (425);  Kood. 

113.  The  Blind  Spot.  Mariotte's  Experiment.  The  point 
of  entrance  of  the  optic  nerve  is  unprovided  with  visual 
end-organs  and  is  irresponsive  to  light.  This  insensitive- 
ness  is  easily  demonstrated  with  the  diagrams  below. 


a.  Close  the  left  eye,  and  keeping  the  right  fixed  on  the 
upper  asterisk  in  the  diagram  move  the  latter  toward  the 
eye  and  away  from  it  till  a  point  is  found  where  the  black 
oval  disappears.  For  the  blind  spot  of  the  left  eye,  turn 
the  diagram  upside  down  and  close  the  right  eye. 

The  blind  spot  may  be  demonstrated  simultaneously  in 
both  eyes  with  the  figure  on  the  next  page.  The  experi- 
menter should  look  at  the  asterisk  while  he  holds  a  card 


THE  MECHANISM  OF  THE  EYE.  103 

in  the   median  plane   of  his   head,  to   prevent  each  eye 
from  seeing  the  other's  part  of  the  diagram. 

O  O 

b.  To  draw  the  projection  of  the  blind  spot,  arrange  a 
head-rest  opposite  a  vertical  sheet  of  white  paper,  and  15  or 
18  inches  distant  from  it.  Put  a  dot  on  the  paper  for  a 
fixation  point.  Fasten  upon  the  end  of  a  light  rod  a  bit  of 
black  paper  about  2  mm.  square  or  blacken  the  end  of  the 
rod  with  ink.  Bring  the  face  into  position,  close  one  eye, 
and  fix  the  other  upon  the  dot.  Move  the  rod  slowly  so  as 
to  bring  the  little  square  over  the  part  of  the  paper  corre- 
sponding to  the  blind  spot,  dotting  on  the  paper  the  points 
where  the  square  disappears  or  reappears.  Repeat  at  vari- 
ous points  till  the  outline  of  the  projection  of  the  blind  spot 
is  complete.  If  the  mapping  is  carefully  carried  out,  the 
map  will  probably  also  show  the  points  of  departure  of  the 
large  blood-vessels  that  enter  with  the  nerve. 

Helmholtz,  A,  250-254,  Fr.  284-289  (210-214). 


114.  The  Filling-out  of  the  Blind  Spot  is  of  considerable 
psychological  interest.  The  mind  supplies  what  is  lacking 
in  the  sense,  and  in  doing  so  is  influenced  both  by  the  sensa- 
tions of  the  parts  of  the  retina  surrounding  the  spot  and 
by  previous  experience.  In  ordinary  two-eyed  vision  the 
blind  spot  of  one  eye  corresponds  to  a  seeing  spot  in  the 
other,  and  this  with  the  movements  of  the  eyes  amply  sup- 
plies the  defect.  The  spot,  furthermore,  lies  so  far  out 
of  the  range  of  clear  vision  that  its  existence  is  habitually 
overlooked,  even  in  monocular  vision. 

a.   When  the  image  of  the  oval  in  a  of  the  last  experi- 


104        LABORATORY  COURSE  IN  PSYCHOLOGY. 

ment  is  brought  wholly  upon  the  spot,  the  paper  seems  an 
unbroken  white,  because  the  adjacent  parts  of  the  retina 
are  stimulated  with  white.  When,  however,  the  diagram 
is  held  a  little  nearer  so  that  the  edge  of  the  black  oval  can 
be  seen,  the  filling  is  part  black  and  part  white. 

b.  The  effect  of  experience  appears  when  the  oval  is 
replaced  by  such  a  figure  as  that  below,  or  any  other  in 
which  the  bars  stand  out  well  from  one  another  and  the 
background. 


When  the  image  of  the  middle  of  this  diagram  falls  upon 
the  blind  spot,  one  bar  will  seem  to  cross  completely  over 
the  other.  Bars  that  cross  are  so  much  more  frequent  in 
experience  than  those  that  are  mitered  together  that  the 
sensations  of  the  adjacent  parts  are  thus  interpreted. 
Skill  in  observation  in  indirect  vision  seems  to  hinder  this 
filling-out  process  somewhat,  probably  by  aiding  in  more 
exact  distinguishing  of  the  character  of  the  sensations 
received.  Both  Helmholtz  and  Aubert  find  themselves 
unable  to  determine  how  the  parts  of  the  figure  resting  on 
the  blind  spot  are  related. 

Helmholtz,  A,  Fr.  734-745  (574-583);  Aubert,  A,  595. 


THE  MECHANISM  OF  THE  EYE.  105 

115.  The  Yellow  Spot,  the  Macula  Lutea.    The  projection 
of  the  yellow  spot  in  the  visual  field  can  be  made  visible  in 
several  ways.     Two  have  already  been  mentioned  in  Ex. 
Ill ;    Others   are   as  Jj^ws :    Close   the   eyes   for   a  few 
seconds  and  then  loolMHough  a  flat-sided  bottle  of  chrome 
alum  solution  at  a  brightly  lighted  surface  or  at  the  clear 
sky.     In  the  blue-green  solution  a  rose-colored  spot  will  be 
seen  which  corresponds  to  the  yellow  spot.     The  light  that 
comes  through  the  chrome  alum  solution  is  chiefly  a  mixture 
of  red  and  green  and  blue.     The  pigment  of  the  yellow  spot 
absorbs  a  portion  of  the  blue  and  green  and  transmits  the 
rest,  which   makes  a  rose-colored   mixture,  to   the  visual 
organs  behind  it.     The  same  can  be  very  beautifully  de- 
monstrated with  violet  or  purple  gelatine  sheets. 

Helmholtz,  A,  Fr.  548-551  (419-421);  Maxwell;  Sachs;  Hering,  C. 

116.  Intermittent  Illumination.     Tl^  region  of  the  yel- 
low spot  can  be  seen,  together  with^many  other   curious 
figures  and  patterns,  when  the  illumination  of  a  single  eye 
is  made  intermittent  by  moving  the  spread  fingers  rapidly 
to  and  fro  before  it.     Something  may  be  seen  when  the 
open  eyes  are  fixed  on  a  uniformly  lighted  surface,  but  more 
when  they  are  turned  with  closed  lids  toward  a  bright  sky 
or  the  sun  itself.     The  figures  probably  differ  in  different 
eyes  and  some  are  beautiful  and  elaborate.     Sometimes  with 
steady  fixation  the  figures  give  place  more  or  less  completely 
to  a  general  streaming  of  fine  particles,  suggesting  the  flying 
specks  of  Ex.  112,  but  finer  and  of  less  regular  course. 
Vierordt  credited  the  appearance  to  the  circulation  of  the 
blood  in  the  retinal  vessels  ;  Helmholtz  is  inclined  to  think 
the  fine  particles  lymph  corpuscles  rather  than  blood  corpus- 
cles.    Similar  phenomena  are  to  be  observed  with  black  and 
white  disks  when  rotated  at  less  speed  than  that  required 
for  uniform  mixing  of  the  black  and  white. 

Helmholtz,  A,  532  f.,  Fr.  502  (381)  f.;  Exner,  F. 


106        LABORATORY  COURSE  IN  PSYCHOLOGY. 


117.  Acuteness  of  Vision,  Minimum  Visibile.  a.  Place 
the  parallel  line  diagram  used  in  Ex.  118  in  a  good  light 
and  walk  backward  from  it  till  the  lines  can  just  no  longer 
be  distinguished  as  separate.  If  ti^^perimeiiter's  eyes  are 
not  normal,  he  should  use  glasse^^Blt  fit  his  eyes  for  dis- 
tinct vision  at  the  distance  requireor  Measure  the  distance 
between  the  eye  and  the  diagram,  and  calculate  the  angle 
whose  apex  lies  in  the  crossing  point  of  the  lines  of  direc- 
tion (about  7.2  mm.  back  of  the  cornea  and  15.6  mm.  in 
front  of  the  retina)  and  whose  base  is  the  distance  from 
the  middle  of  one  line  of  the  diagram  to  the  middle  of  the 
next;  in  this  diagram  1.6  mm.  This  angle  measures  the 
least  visible  extent  when  discrimination  is  involved ;  the 
least  luminous  extent  that  can  still  impress  the  retina  is  far 
smaller,  as  witness  the  visibility  of  the  stars.  On  the  sup- 
position that  if  the  sensations  of  two  cones  are  to  be  separ- 
able they  must  be  s^arated  by  an  unstimulated  cone,  or  at- 
least  by  a  less  stimulated  one,  it  has  generally  been  consid- 
ered that  the  cones  could  not  subtend  a  greater  angle 
than  that  found  in  this  experiment,  60" — 90",  represent- 
ing 0.004 — 0.006  mm.  on  the  retina,  and  this  agrees 
well  with  microscopical  measurements.  But  as  Helm- 
holtz  notices  (Phys.  Opt.,  2d  ed.,  p.  260),  this  experiment 
does  no  more  than  prove  that  there  are  on  the  retina  rows 
of  sensitive  elements,  the  middle  lines  of  which  are  sepa- 
rated by  the  angular  distance  found  in  the  experiment. 
The  elements  themselves,  if  properly  arranged,  may  be 
somewhat  larger.  Calculation  of  the  number  of  such  ele- 
ments in  a  sq.  mm.  of  the  retina,  based  on  this  view  of  the 
experiment,  agrees  well  in  the  case  of  Helmholtz's  own 
determination  with  the  result  of  microscopical  counting. 

b.  The  discriminative  power  of  the  retina  falls  off  rapidly 
in  all  directions  from  the  fovea  —  more  rapidly  above  and 
below  than  in  a  horizontal  direction.  Arrange  a  head-rest 


THE  MECHANISM  OF  THE  EYE. 


107 


and  perpendicular  plane  as  in  Ex.  113,  b  (or  if  a  perimeter 
is  at  hand  use  that).  Place  upon  the  end  of  the  rod  used  in 
that  experiment  a  card  on  which  have  been  made  two  black 
dots  2  mm.  in  diameter  and  4  mm.  from  centre  to  centre. 
Move  the  card  horizontally  toward  the  fixation  point,  begin- 
ning beyond  the  point  at  which  the  two  dots  can  be  dis- 
tinguished and  moving  inward  till  they  can  just  be 
distinguished.  Measure  the  distance  from  the  fixation 
point,  and  repeat  several  times  both  to  the  right  and  left 
of  the  fixation  point,  holding  the  card  so  that  both  dots  are 
in  each  case  equally  distant  from  that  point.  Try  the  same 
for  the  vertical  meridian. 

Helmholtz,  A,  255-264,  Fr.  291-301  (215-223);  Uhthoff.     On  a, 
Aubert,  A,  579-585;  on  6,  585-591.     On  6,  see  also  Exner,  D,  242  ff. 


118.  Bergmann's  Experiment.  Place  the  left  hand  dia- 
gram in  a  good  light,  and  look  at  it  from  a  distance  of  a  yard 
and  a  half  or  two  yards.  Observe  the  apparent  bending  and 
beading  of  the  lines.  This  is  believed  by  Helmholtz  to  be  due 
to  the  mosaic  arrangement  of  the  visual  cones.  The  cones 
that  are  touched  by  the  image  of  one  of  the  white  lines  are 
stimulated  in  proportion  as  they  are  more  or  less  touched. 
Those  that  are  much  stimulated  furnish  the  sensation  of 
the  white  line  and  its  irregularities;  those  that  are  little 


108        LABOEATOEY  COUESE  IN  PSYCHOLOGY. 

stimulated  join  with  those  that  are  not  touched  at  all  to 
give  the  image  of  the  black  line  and  its  irregularities. 
This  is  schematically  represented  in  the  right  hand  cut. 
Von  Fleischl,  on  the  other  hand,  has  made  experiments  to 
show  that  the  bending  and  beading  of  the  lines  is  not  con- 
nected with  the  retinal  mosaic,  but  rather  with  movements 
of  the  eyes  that  sweep  the  point  of  fixation  backward  and 
forward  across  the  lines.  Further  than  this  his  explanation 
does  not  go. 

Helmholtz,  A,  257-258,  Fr.  293  294  (217-218);  von  Fleischl. 

119.  Mechanical  Stimulation  of  the  Retina,  a.  Phos- 
phenes.  Turn  the  open  or  closed  eye  as  far  as  possible 
toward  the  nose  and  press  on  the  eyelid  at  the  outer  corner 
with  the  finger  or  the  tip  of  a  penholder.  On  the  opposite 
side  of  the  visual  field  will  be  seen  a  more  or  less  complete 
circle  of  light  surrounded  by  a  narrow  dark  band,  outside  of 
which  again  is  a  narrow  band  of  light.  Notice  the  color  of 
the  light  seen.  Get  phosphenes  by  pressure  at  other  points 
of  the  eyeball. 

b.  Press  the  eye  moderately  with  some  large  object,  say, 
the  angle  of  the  wrist  when  the  hand  is  bent  backward,  and 
continue  the  pressure  for  a  minute  or  two.     Peculiar  palpi- 
tating figures  will  be  observed  and  strange  color  effects. 
The  former  Helmholtz  compares  to  the  tingling  of  a  mem- 
ber that  is  "  asleep." 

c.  Standing  before  a  window,  close  the  eyes  and  turn 
them  sharply  from  side  to  side.     As  they  reach  the  extreme 
position  in  either  direction,  observe  immediately  in  front  of 
the  face  a  sudden  blue  spot  surrounded  by  a  yellow  band. 
A  second  fainter  spot  farther  from  the  centre  in  the  direc- 
tion of  motion  may  also  be  seen.     The  appearance  of  the 
first  spot  is  due  to  a  mechanical  stimulation  of  a  portion  of 
the  retina  at  the  edge  of  the  blind  spot  in  the  eye  that  turns 


THE  MECHANISM  OF  THE  EYE.  109 

inward.     The  second  spot  belongs  to  the  corresponding  area 
in  the  other  eye. 

Helmholtz,  A,  235-239,  Fr.  266-270  (196-200),  744  (583)  f. 

120.  Idio-retinal  Light,  Light  Chaos,  Light  Dust.    a.  Close 
and  cover  the  eyes  so  as  to  exclude  all  light,  taking  care 
not  to  press  them,  or  experiment  in  a  perfectly  dark  room. 
Let  the  after-effects  of  objective  light  fade  away,  and  then 
watch  the  shifting  clouds  of  retinal  light.     The  cause  of 
the  retinal  light  is  not  altogether  clear,  but  it  is  supposed 
to  be  a  chemical  action  of  the  blood  on  the  nervous  portion 
of  the  visual  apparatus.     Aubert  estimates  its  brightness 
at  about  half  the  brightness  of  a  sheet  of  paper  illuminated 
by  the  planet  Venus  when  at  its  brightest. 

b.  When  awake  in  the  night  time  in  a  room  that  is  almost 
perfectly  dark  (e.g.,  in  which  the  form  of  the  window  and 
the  large  pieces  of  furniture  cannot  be  made  out),  notice 
that  the  white  clothing  of  the  arms  can  be  seen  faintly 
when  they  are  moved  about,  but  not  when  they  are  still.  In 
the  last  case  the  very  faint  light  they  reflect  is  not  sufficient 
to  make  them  distinguishable  from  clouds  of  idio-retinal 
light. 

Helmholtz,  A,  242-243,  Fr.  274-275  (202-203).  On  6,  Helm- 
holtz, jB. 

121.  Electrical    Stimulation  of   the   Visual    Apparatus. 
Moisten  thoroughly  with  salt  water  both  the  electrodes  and 
the  portions  of  the  skin  to  which  they  are  to  be  applied. 
Place  one  of  the  electrodes  on  the  forehead  (or  on  the  edge 
of  the  table  and  lay  the  forehead  upon  it),  the  other  on  the 
back  of  the  neck ;  or,  if  the  current  is  strong  enough,  hold 
it  in  the  hand  or  lay  it  on  the  table  with  the  hand  upon  it. 
At  each  opening  or  closing  of  the  circuit,  a  bright  flash  will 
be  seen,  whether  the  eyes  are  closed  or  open.   With  the  eyes 
closed  and  covered,  the  effects  of  the  continuous  current 


110       LABORATORY  COURSE  IN  PSYCHOLOGY. 

may  be  observed.  In  this  case  it  is  well  to  apply  the  elec- 
trode slowly  and  carefully  so  as  to  avoid  as  much  as  possi- 
ble the  flash  caused  by  the  sudden  closing  of  the  circuit. 
When  the  positive  electrode  is  on  the  forehead,  the  nega- 
tive on  the  back  of  the  neck,  a  transient  pale  violet  light 
will  be  seen  distributed  generally  over  the  field  and  forming 
a  small  bright  spot  at  its  centre.  Sometimes  traces  of  the 
blind  spot  also  appear.  The  violet  light  soon  fades,  and  on 
opening  the  circuit  there  is  a  notable  darkening  of  the 
field,  with  a  momentary  view  of  the  blind  spots  as  bright 
disks.  When  the  negative  electrode  is  on  the  forehead,  the 
positive  on  the  back  of  the  neck,  the  phenomena  are  in 
general  reversed,  the  darkening  occurring  on  closing  the  cir- 
cuit, the  violet  light  on  opening  it.  Helmholtz  sums  up 
these  and  other  experiments  in  the  following  law :  "  Con- 
stant electrical  circulation  through  the  retina  from  the 
cones  toward  the  ganglion  cells  gives  the  sensation  of 
darkness ;  circulation  in  the  contrary  direction  gives  the 
sensation  of  brightness."  (Phys.  Opt.,  2d  ed.,  p.  247.)  That 
the  blind  spot  should  appear  as  a  disk  of  different  color 
from  the  rest  of  the  field  seems  to  be  due  to  the  fact  that 
the  sensitive  parts  of  the  retina  immediately  surrounding 
it  are  somewhat  shielded  from  the  electric  current,  and  as 
usual  their  condition  is  attributed  to  the  blind  spot  also. 
The  experiment  is  not  altogether  a  pleasant  one,  on  account 
of  the  feeling  which  the  current  produces  in  the  head,  the 
"  electrical  taste  "  in  the  mouth,  and.  the  reddening  of  the 
skin  under  the  electrodes. 

Helmholtz,  A,  243-248,  Fr.  275-281  (203-207),  744  (583). 

RETINAL  FATIGUE  AND  ADAPTATION. 

122.  Eetinal  Fatigue.  Stare  with  perfectly  fixed  and 
motionless  eyes  at  a  selected  spot  on  a  variegated  carpet  or 
wall  paper,  and  notice  the  levelling  effect  of  fatigue.  The 


THE  MECHANISM  OF  THE  EYE.  Ill 

differences  in  color  and  pattern  gradually  disappear,  and 
the  whole  field  becomes  a  nearly  uniform  cloud.  The  parts 
of  the  recina  that  are  strongly  stimulated  are  brought 
down  to  the  general  level ;  those  that  are  little  stimulated 
are  built  up  to  it.  Every  wink  or  slight  movement  of  the 
eyes  causes  a  general  brightening  up  of  the  field  and 
restoration  of  vision.  The  experiment  is  particularly  easy 
to  make  when  looking  at  a  uniform  surface  with  faint 
shadows  lying  on  it. 

Helmholtz,  A,  508,  555  ff.,  Fr.  478  (362),  527  (402)  ff.;  Fick,  B, 
222;  Treitel;  Hering,  C.  See  also  the  discussion  on  this  topic  by 
A.  E.  Fick  and  Hering. 

123.  Adaptation  of  the  Eye.  a.  The  adjustment  of  the 
eye  to  the  intensity  of  its  illumination  is  effected  partly  by 
change  in  the  size  of  the  pupil,  and  partly  by  changes  in 
the  retina  itself.  The  first  is  of  common  observation,  and 
the  connection  of  the  two  eyes  in  this  respect  has  been 
noticed  in  Ex.  110,  b.  The  effects  of  going  from  a  dark 
room  into  a  light  room  and  vice  versa,  and  the  gradual  im- 
provement of  vision  on  remaining  in  one  or  the  other,  are 
also  familiar. 

b.  It  has  not,  however,  been  so  generally  observed  that 
adaptation  to  very  weak  lights  is  much  more  favorable  to 
the  perception  of  colorless  light  than  to  colored.  This  may 
easily  be  observed  in  a  dark  room  with  single  flashes  of  a 
rather  faint  Geissler  tube.  Before  the  room  is  darkened, 
and  for  a  short  time  after,  the  colors  of  the  light  are  readily 
perceived.  After  some  time,  however,  they  nearly  or  quite 
fail,  seeming  to  be  lost  in  the  increased  brilliancy  of  the 
white  light.  It  is  important  that  there  should  be  an  inter- 
val between  the  flashes  sufficient  to  allow  all  the  effects  of 
one  to  disappear  before  another  is  given.  If  the  room  is 
not  completely  dark,  the  head  of  the  observer  and  the  tube 


112       LABORATORY  COURSE  IN  PSYCHOLOGY. 

must  be  covered  closely  with  an  opaque  cloth  to  allow  full 
adaptation. 

Aubert,  A,  483  f.,  B,  25  ff. ;  Charpentier,  A,  154  ff. ;  Treitel;  Bering, 
C.  On  6,  Hillebrand. 

AFTER-IMAGES. 

After-images,  Accidental  or  Consecutive  Images.  After- 
images in  which  the  relations  of  light  and  shade  of  the 
original  object  are  preserved  are  called  Positive  After-images. 
Those  in  which  these  relations  are  reversed  (as  in  a  photo- 
graphic negative)  are  called  Negative  After-images.  Posi- 
tive after-images  are  of  various  colors,  but  most  important 
to  notice  here  are  those  of  the  color  of  the  object  (like- 
colored),  and  of  the  complementary  color  (opposite-colored). 
Negative  after-images,  so  far  as  observed,  are  always  oppo- 
site-colored. All  after-images,  especially  the  positive,  can 
best  be  observed  in  the  morning  when  the  eyes  are  well 
rested. 

124.  Negative  After-images,  a.  Look  steadily  for  a  minute 
at  a  fixed  point  of  the  window,  then  at  a  white  screen  or  an 
evenly  lighted,  unfigured  wall ;  the  dark  parts  of  the  win- 
dow will  now  appear  light  and  the  light  dark. 

b.  Get  a  lasting  after-image  and  look  at  a  corner  of  the 
room,  or  at  a  chair  or  other   object   of  uneven   surface  ; 
notice  how  the  image  seems  to  fit  itself  to  the  surface  upon 
which  it  rests.     After  a  little  practice  it  is  also  possible  at 
will  to  see  the  image  floating  in  the  air  instead  of  lying  on 
the  background. 

c.  Look  steadily  at  a  bright-colored  object  or  some  bits 
of  colored  paper,  then  at  the  screen ;  observe  that  the  colors 
of  the  after-images  are  approximately  complementary  to  the 
colors  of  the  objects  producing  them. 

d.  Negative  After-images  upon  a  Background  faintly  Tinged 
with  the  Stimulating  Color.     Fasten  upon  the  color-mixer  a 


THE  MECHANISM  OF  THE  EYE.  118 

white  disk  upon  which  has  been  painted  a  six  rayed  star  of 
red.  Set  the  disk  in  rapid  rotation,  bring  the  eyes  within 
eight  or  ten  inches  of  the  disk,  and  after  half  a  minute  sud- 
denly withdraw  them  to  thirty  or  forty  inches.  As  the 
head  is  drawn  back  the  complementary  color  will  be  seen 
to  press  in  upon  the  disk  from  all  sides  while  the  red  con- 
tracts. When  the  head  is  again  approached  to  the  disk  the 
red  will  enlarge  and  the  blue-green  disappear.  The  cause 
of  the  rushing  in  of  the  blue  in  the  first  case  is  the  contrac- 
tion of  the  retinal  image,  which  of  course  decreases  in  size 
as  the  head  is  drawn  back,  and  is  thus  brought  upon  parts 
of  the  retina  that  have  been  more  strongly  stimulated. 
When  the  head  approaches  the  disk  the  retinal  image 
enlarges  and  its  outer  portion  lies  on  a  fresh  area.1 

Negative  after-images  are  sometimes  very  lasting,  and  for 
that  reason  are  those  most  frequently  noticed  in  ordinary 
experience.  They  are  phenomena  of  retinal  fatigue  (Helm- 
holtz),  or  of  retinal  restitution  (Hering). 

125.  Positive  After-images.  These  images  are  not  diffi- 
cult to  see,  if  after  a  brief  stimulation  the  eye  is  shielded 
from  further  action  of  light.  Thus,  when  the  gas  is  sud- 
denly turned  off  in  a  dark  room,  the  positive  image  of  the 
flame  and  the  burner  is  very  easily  seen. 

a.  Look  for  an  instant  (one-third  of  a  second)  at  the  win- 
dow, then  close  and  cover  the  eyes.  Notice  that  the  after- 
image is  like  the  window  in  distribution  of  light  and  shade, 
bright  panes  and  dark  bars,  and  at  first  like  it  also  in  color. 
After  some  practice  it  is  also  possible  to  see,  for  a  small 
fraction  of  a  second,  the  positive  after-image  of  almost  any 
bright  object  on  suddenly  turning  the  eyes  from  the  object 
to  some  other  part  of  the  field,  especially  if  the  latter  is 
dark.  The  positive  after-image  is  of  short  duration  and 
less  readily  observed  than  the  negative.  It  has  generally 

1  For  a  still  simpler  experiment,  see  Mind,  Ser.  2,  IT.,  1893,  485,  note. 


114       LABORATORY  COURSE  IN  PSYCHOLOGY. 

been  considered  a  phenomenon  of  retinal  inertia,  a  prolon- 
gation of  the  original  retinal  excitation,  and  such  a  prolonga- 
tion does  undoubtedly  exist.  Charpentier  and  Hess,  however, 
in  experiments  with  very  brief  stimulation,  have  found 
a  transient  negative  image  coming  between  the  original 
impression  and  the  ordinary  positive  after-image  observed 
with  longer  stimulation.  The  full  series  would  then  be : 
1.  Prolongation  of  the  original  stimulus ;  2.  First  Negative 
Image ;  3.  Ordinary  Positive  After-image ;  4  Ordinary 
Negative  After-image. 

b.  Colored  Positive  After-images.     Look  for  an  instant  at 
a  gas  flame  through  a  piece  of  red  glass,  then  close  and 
cover  the  eyes  and  observe  the  red  image ;  repeat  the  exper- 
iment, continuing  the  fixation  of  the "  flame  for  half  a  min- 
ute ;  the  resulting  after-image  will  be  bright  as  before  but 
of  the  opposite  color. 

c.  After-images  on  Dark  and  Light  Backgrounds.      Get 
an  after-image  of  the  window  of  not  too  great  intensity,  and 
project  it  alternately  on  a  sheet  of  white  paper  and  the  dark 
field  of  the  closed  and  covered  eyes ;  it  will  be  found  nega- 
tive on  the  white  background  and  positive  on  the  dark. 
Some  observers  find  a  periodic  reappearance  of  positive 
after-images,  or  an  alternation  of  positive  and  negative 
images,  without  a  change  of  background. 

d.  Sequence  of  Colors.      Get  a  good  after-image  of  the 
window,  and  observe  with  closed  and  covered  eyes  the  play 
of  colors  as  the  image  fades.     Try  several  times  and  observe 
that  the  order  of  succession  is  the  same.      According  to 
Hering,  this  play  of  colors  would  not  take  place  if  the  origi- 
nal stimulus  were  absolutely  colorless. 

On  Exs.  124  and  125,  consult  the  following:  Helmholtz,  A,  480  ff., 
501  ff.,  Fr.  446  (338),  471-500  (357-380);  Wundt,  A,  3te  Aufl.,  I., 
472-476,  4te  Aufl.,  I.,  512  ff.;  Hess;  Charpentier,  B.  See  also  ref- 
erences given  in  Chap.  VI.  for  Successive  Contrast. 


THE  MECHANISM  OF  THE  EYE.  115 

126.  Effect  of  Eye-motions  on  After-images.     Get  a  mod- 
erately strong  after-image  of  the  window  ;  look  at  the  wall 
and  keep  the  eyes  actively  in  motion.     The  image  will  be 
seen  with  difficulty  while  the  eye  is  in  motion  ;  when,  how- 
ever, the  eye  is  brought  to  rest,  it  will  soon  appear.     In 
general,  any  visual  stimulus  that  moves  with  the  eye  is  less 
effective  than  one  that  does  not. 

Exner,  A. 

127.  The  Seat  of  the  After-image.     An  after-image  due  to 
stimulation  of  one  eye  may,  under  proper  conditions,  some- 
times seem  to  be  seen  with  the  other.     From  this  it  has 
been  inferred  that  the  seat  of  after-images  is  central,  not 
peripheral ;   that  is,  in  the   visual   centres   of  the   brain, 
higher  or  lower,  not  in  the  retina.     The  following  experi- 
ments show,  however,  that  the  after-image  is  really  seen 
with  the  eye  first  stimulated,  and  so  render  the  hypothesis 
of  a  central  location  unnecessary. 

a.  Look  steadily  for  a  considerable  time  at  a  bit  of  red 
paper  on  a  white  ground,  using  only  one  eye,  say  the  right, 
and  keeping  the  other  closed;  when  a  strong  after-image 
has  been  secured,  remove  the  paper,  close  the  right  eye, 
open  the  left,  and  again  look  steadily  at  a  fixed  point  on  the 
white  ground ;  after  a  little  the  field  will  darken  and  the 
after-image  will  reappear.     If  the  red  does  not  produce  a 
sufficiently  lasting  image,  substitute  for  it  a  gas  flame  or 
some  other  bright  object. 

b.  That  we  have   really  to  do  with  the   eye   originally 
stimulated  (its  present  dark  field  suppressing  the  light  one  of 
the  other  eye),  appears  from  such  experiments  as  the  fol- 
lowing :    Get  the   after-image  as  before ;    then   open  both 
eyes   and  bring  a  bit  of  cardboard  before  the  eyes  alter- 
nately.    Bringing  it  before  the  left  eye  rather  brightens  the 
image ;  bringing  it  before  the  right  dims  or  abolishes  it. 


116        LABORATORY  COURSE  IN  PSYCHOLOGY. 

The  image  is  thus  chiefly  affected  by  what  affects  the  right 
eye. 

c.  Get  the  after-image  again,  and  close  and  cover  both 
eyes ;  observe  the  color  of  the  after-image,  as  projected  on 
the  dark  field ;  then  open  the  left  eye,  letting  the  right  eye 
remain  closed  and  covered.  The  after-image  will  be  seen, 
not  in  the  color  it  has  when  the  right  eye  is  open  and  the 
image  is  projected  in  the  light  field,  but  in  that  which  it 
has  in  the  dark  field  of  the  closed  eye. 

These  experiments  prove  that  after-images  belong  to  the 
stimulated  half  of  the  visual  apparatus,  but  they  do  not 
show  whether  the  images  belong  to  the  retina  of  that  half 
or  to  the  nervous  centres  connected  with  it.  Other  consid- 
erations, such,  for  example,  as  the  fact  that  the  image  fol- 
lows every  motion  of  the  eye,  even  those  that  are  usually 
unconscious,  is  affected  by  pressures  exerted  on  the  eyeball 
and  by  electric  currents  sent  through  it,  together  with  Ex- 
ner's  direct  experiments  on  retinal  and  optic  nerve  stim- 
ulation, support  the  retinal  location,  in  favor  of  which 
current  opinion  is  practically  unanimous.  Some  observers, 
however,  have  been  able  to  get  a  binocular  after-image  of 
a  somewhat  different  character ;  see  binocular  section  of 
Chap.  VI. 

Delabarre  ;  Exner,  D,  246  ff.  and  E ;  Fick  and  Giirber,  296  ff. 

128.  After-images  of  Motion.  These  after-images  can  be 
secured  from  almost  any  continuously  moving  object.  They 
are  often  unpleasantly  striking  after  looking  at  the  water 
from  the  deck  of  a  vessel  or  at  the  landscape  from  a  car 
window.  In  the  experiments  below,  variations  of  one  of 
the  laboratory  methods  of  producing  them  are  given. 

a.  Fasten  upon  the  rotation  apparatus  a  disk  bearing  a 
large  number  of  equal  black  and  white  sectors ;  set  it  in 
slow  rotation  and  gaze  fixedly  at  it.  The  rate  must  not 


TlfE  MECHANISM  OF  THE  EYE.  117 

be  fast  enough  to  blur  the  outlines  of  the  sectors  very 
much.  After  a  moment  or  two  of  steady  fixation,  bring  it 
suddenly  to  rest  and  observe  its  slow  illusory  backward 
movement. 

b.  Fasten  on  the  apparatus  a  disk  like  that  in  the  accom- 
panying cut,  and  get  an  after-image  as  before,  fixating  the 
centre.    Bring  the  disk 

suddenly    to    rest,    or 

look  away  from  it  to  a 

page  of   print  or  into 

the  f a,ce  of  a  bystander 

and  notice  the  apparent 

shrinking  or  swelling, 

reversing  the  previous 

motion  of   the   spiral. 

Illusions    of    increase 

or  decrease  of  distance 

sometimes    accompany 

those   of   motion  with 

this  disk.     Eepeat  the  experiment,  but  this  time  instead  of 

looking  at  some  object,  close  the  eyes  and  turn  them  toward 

the  sky  or  other  source  of  bright  light.    The  apparent  motion 

will  be  observed  again  in  the  red-yellow  field. 

c.  Hold  over  half  of  the  disk  while  in  rotation  a  piece  of 
cardboard,  fixate  the  centre  of  the  disk,  and  get  the  after- 
image.     Observe  that  the  after-image   is   limited  to  the 
portion  of  the  retina  stimulated. 

d.  Get  a  monocular  after-image  of  the  spiral,  with  the 
right  eye,  for  example.     Then  close  the  right  eye  and  open 
the  left ;  the  after-image  of  motion  will  be  projected  like 
that  of  color  in  Ex.  127. 

e.  Hold  just  above  the  spiral  disk  a  larger  disk  of  paste- 
board, cut  with  a  radial  slot  an  inch  or  two  wide.     When 
the  spiral  is  now  revolved  a  narrow  strip  will  be  seen  in 


118        LABORATORY  COURSE  IN  PSYCHOLOGY. 

which  the  motion  is  in  one  direction  only.  Get  a  strong 
after-image  and  observe  it  with  closed  eyes  as  in  b  above. 
It  will  sometimes  be  possible,  at  least  for  a  short  time,  to 
get  a  reversal  of  the  previous  illusion ;  the  part  of  the  image 
corresponding  to  the  slot  will  appear  to  stand  still  while  the 
adjacent  parts  move,  or  both  will  appear  in  motion  in  op- 
posite directions.  This  experiment  is  apparently  easier 
to  get  with  the  antirrheoscope,  where  the  moving  field  is 
larger.  With  that  instrument  the  effect  mentioned  can  be 
seen  in  the  ordinary  projected  after-image. 

When  a  strong  after-image  is  projected  upon  a  set  of 
straight  lines  at  right  angles  to  the  direction  of  movement, 
some  observers  have  seen  the  lines  more  or  less  distorted  by 
it  (Budde  saw  them  thus  affected  when  the  lines  did  not 
cross,  but  only  entered  the  moving  part  of  the  field)  ;  others 
have  found  the  lines  entirely  unaffected.  It  seems  prob- 
able that  the  breadth  and  distinctness  of  the  lines  have 
something  to  do  with  this  difference  of  results. 

Exner,  who  believes  in  the  retinal  seat  of  color  after- 
images, is  inclined  to  give  a  more  central  location  to  these 
of  motion.  In  his  opinion  such  experiments  as  those  above 
indicate  also  that  our  knowledge  of  such  motions  is  a  sensa- 
tion, not  a  perception. 

After-images  of  motion  have  been  explained  by  actual, 
though  unconscious,  movements  of  the  eyes,  like  the  ap- 
parent movements  of  objects  in  dizziness.  This  is  certainly 
incorrect ;  for  in  b  it  would  seem  necessary  that  the  eyes 
should  move  in  all  directions  at  once,  and  c  shows  that  the 
effect  is  limited  to  a  portion  of  the  field,  which  would  be 
impossible  if  it  were  due  to  actual  eye  motions.  The  same 
was  demonstrated  by  Dvorak  by  means  of  a  disk  with  three 
concentric  spirals,  the  inner  and  outer  ones  being  drawn 
in  the  same  way,  (right-handed  spirals,  for  example),  while 
that  between  was  drawn  in  the  reverse  direction.  How  far 


THE  MECHANISM  OF  THE  EYE.  119 

some  psychical  representation  of  ocular  motions  co-operates 
in  the  illusion  would  be  hard  to  say. 

Helmhojtz,  A,  Fr.  766-769  (603-605);  Bowditch  and  Hall;  Mach, 
A,  59-61  (see  also  61-65  for  yet  another  kind  of  after-image),  and  I?, 
65-67;  Exner,  B  and  O,  440  ff.;  Dvorak;  Budde;  von  Fleischl; 
Heuse;  Zehfuss. 

MOVEMENTS  OF  THE  EYES. 

The  eye  is  a  moving  as  well  as  a  seeing  member ;  and  its 
motor  functions  are  of  great  importance  for  psychology,  es- 
pecially for  the  theory  of  the  visual  perception  of  space. 
The  experiences  of  the  eye  in  motion  have  a  controlling 
influence  upon  its  perceptions  even  when  at  rest,  as  will 
appear  in  some  of  the  experiments  of  Chap.  VII. 

All  motions  of  the  eye  may  be  conceived  as  rotations  of 
greater  or  less  extent  about  one  or  more  of  three  axes  :  a 
sagittal  axis,  corresponding  nearly  with  the  line  of  sight ; 
a  frontal  axis,  extending  horizontally  from  right  to  left ;  and 
a  vertical  axis.  Theoretically  all  these  intersect  at  right 
angles  in  the  Centre  of  Rotation  of  the  eye.  As  a  land- 
mark from  which  to  measure  eye-movements,  that  position 
(approximately)  is  taken  which  the  eyes  assume  when  the 
head  and  body  are  erect  and  the  eyes  are  directed  forward 
to  a  distant  horizon.  This  is  known  as  the  Primary  Posi- 
tion of  the  eyes  (or  the  lines  of  sight) ;  any  other  is  a 
Secondary  Position.  The  point  on  which  the  eyes  are  fixed 
when  in  the  primary  position  is  the  Primary  Fixation 
Point,  or  Principal  Point  of  Regard.  The  Field  of  Vision 
is  the  extent  of  space  that  can  be  seen  with  the  eye  at 
rest.  The  Field  of  Regard  is  the  extent  of  space  that  can 
be  seen  when  the  eyes  are  moved.  In  the  following  experi- 
ments the  word  Rotation,  except  in  the  expression  "  centre 
of  rotation,"  is  reserved  for  turnings  about  the  sagittal 
axis. 


120        LABORATORY  COURSE  IN  PSYCHOLOGY. 

129.  Reflex  Movements  of  the  Eye.     Of  the  first  impor- 
tance among  eye  movements  is  the  constant  reflex  tendency 
of  the  eye  to  move  in  such  a  way  as  to  bring  any  bright 
image  lying  on  a  peripheral  part  of  the  retina,  or  any  to 
which  attention  is  directed,  into  the  area  of  clearest  vision. 
Many   evidences   of  this   tendency  will  be  found  in  the 
ordinary  course  of  vision.     By  way  of  experiment,  try  to 
study  attentively  a  musca  volitans  or  a  negative  after-image 
that  is  just  to  one  side  of  the  direct  line  of  sight.     The 
apparent  motion  of  the  object  measures  the  energy  of  the 
reflex. 

130.  Associated  Movements  of  the  Eyes.     The  two  eyes 
form  a  single  visual  instrument;  and  even  when  one  eye 
is  closed,  it  follows  to  a  considerable  degree  the  movements 
of  its  open  companion.     Movements  upward  or  downward 
in  normal  vision  are  always  performed  simultaneously  by 
the  two  eyes. 

a.  Close  one  eye,  and,  resting  the  finger-tip  lightly  on  the 
lid,  feel  the  motions  of  that  eye  as  the   other  looks  from 
point  to  point  of  the  field  of  regard. 

b.  Get  a  monocular  after-image,  as  in  Ex.  127,  and  when 
it  seems  visible   to    the  open   eye,   notice  that   it   accom- 
panies the  fixation  point  of  that  eye  as  it  moves  from  point 
to  point  of  the  field  of  regard. 

Aubert,  A,  651  ff. ;  Bering,  A,  519  ff. 

131.  Motions  of  the  Eyes  when  the  Lines  of  Sight  are 
Parallel.     The   movements  here   considered  are  somewhat 
simplified  for  easier  exposition. 

a.  Donders's  Law  ;  the  Law  of  ^Constant  Orientation 
(Helmholtz)  ;  the  Law  of  Like  Position  with  Like  Direction 
(Hering).  It  is  evident  that  when  the  eye  is  fixed  upon  some 
point  of  its  field,  e.g.,  ten  degrees  upward  and  fifteen  degrees 
to  the  right  of  the  primary  position,  it  is  not  thereby  fixed 


THE  MECHANISM  OF  THE  EYE.  121 

as  regards  its  sagittal  axis,  but  might  conceivably  assume 
an  indefinite  number  of  positions  by  different  degrees  of  rota- 
tion about,  that  axis.  It  might  also,  if  not  entirely  free  in 
its  rotation,  rotate  now  through  one  angle  and  now  through 
another,  depending  on  the  direction  in  which  the  line  of 
sight  had  moved  to  reach  the  position  in  which  it  is  then 
found.  As  a  matter  of  fact,  however,  it  does  not  assume 
an  indefinite  number  of  positions,  but  one  and  only  one,  no 
matter  by  what  movements  the  line  of  sight  has  come  to 
that  point.  This  is  Donders's  Law;  and  the  fact  that  it 
expresses  is  of  importance  for  sure  and  easy  recognition  of 
directions  in  the  field  of  regard,  and  for  deciding  whether 
or  not  objects  in  the  field  have  moved  when  the  eye  itself 
has  been  moved.  The  correctness  of  this  law  is  easy  to 
demonstrate. 

Cut  in  a  sheet  of  black  cardboard  two  slits  an  eighth  of 
an  inch  wide  and  four  or  five  inches  long,  crossing  at  right 
angles.  Set  the  cardboard  in  the  window  or  before  some 
other  brightly  lighted  surface.  Arrange  a  head-rest  at  a 
considerable  distance,  and  when  the  head  is  in  position,  get 
a  strong  after-image  of  the  cross,  fixating  its  middle  point. 
Then,  without  moving  the  head,  turn  the  eyes  to  different 
parts  of  the  walls  and  ceiling.  The  image  will  suffer 
various  distortions  from  the  different  surfaces  upon  which  it 
is  projected,  but  each  time  the  eye  returns  to  the  same  point 
the  image  will  lie  as  before.  If  the  wall  does  not  offer  fig- 
ures by  which  this  can  be  determined,  have  an  assistant 
mark  the  position  of  the  image  upon  it.  The  after-image  is 
of  course  fixed  on  the  retina  and  can  move  only  as  the  eye 
moves. 

b.  Listing's  Law.  This  law  goes  beyond  Donders's  Law, 
and  asserts  that  the  position  is  not  only  fixed,  but  that  in 
movements  from  the  primary  position  there  is  no  rotation 
at  all  about  the  sagittal  axis.  In  other  words,  the  final  posi- 


122        LABORATORY  COURSE  IN  PSYCHOLOGY. 

tion  is  such,  as  the  eye  would  assume  if  it  were  moved  from 
its  primary  position  to  the  position  in  question  by  turning 
about  a  fixed  axis  standing  perpendicular  at  the  centre  of 
rotation  to  both  the  primary  and  the  new  position  of  the 
line  of  sight.  To  show  this  requires  a  little  more  care  than 
the  last  experiment. 

The  observer  must  be  placed  at  a  distance  of  twenty-five 
or  thirty  feet  from  an  extensive  wall  space,  with  a  suitable 
head-rest  as  before.  The  lines  of  sight  are,  of  course,  not 
strictly  parallel  at  this  distance,  but  the  difference  may  be 
neglected.  On  the  wall  stretch  dark-colored  strings  as  indi- 
cated in  the  accompanying  diagram.  The  cross  at  the  lower 
right  hand  corner  should  be  approximately  in  the  primary 
position  for  the  observer.  The  longer  vertical  and  horizontal 
strings  should  be  twelve  or  fifteen  feet  long,  the  inclined 
one  eighteen  or  twenty  feet.  The  angle  that  the  last  makes 
with  the  others  is  not  important  so  long  as  it  is  not  too 

small  with  either.  Fix- 
ation points  of  black 
cardboard  or  some  other 
conspicuous  substance 
should  be  affixed  as  indi- 
cated by  the  little  circles. 
The  cross  in  the  corner 
may  be  made  by  pasting 
strips  of  bright-colored 
paper  half  an  inch  wide 
and  a  foot  long  on  a 
disk  of  white  c  a  r  d- 
board,  or  (better  still)  it 
may  be  made  by  the  line 
of  junction  of  four  colored  sectors,  two  red  arid  two  blue,  for 
example.  The  disk  in  either  case  must  be  so  arranged  that 
it  can  be  turned  about  its  centre  and  one  of  its  diameters 


TEE  MECHANISM  OF  THE  EYE.  123 

be  made  to  coincide  with  the  oblique  string.  When  all  has 
been  arranged  make  the  following  tests  :  — 

Exact  determination  of  the  primary  position.  For  most 
observers  this  is  somewhat  depressed  below  the  horizontal 
position.  Let  the  observer  fixate  the  centre  of  the  disk 
till  he  has  secured  a  strong  and  clear-cut  after-image  of  it 
and  then  turn  his  eyes,  taking  care  not  to  move  his  head, 
to  the  fixation  marks  on  the  horizontal  and  vertical  strings. 
If  the  corresponding  lines  of  the  after-image  coincide  with 
the  strings,  the  head  is  in  the  required  position.  If  not,  the 
head  must  be  moved  a  little  to  right  or  left  if  the  error 
is  with  the  vertical  bar,  and  up  or  down  if  with  the  hori- 
zontal. The  primary  position  differs  a  little  from  observer 
to  observer,  and  even  with  the  same  observer  at  different 
times. 

Having  found  the  primary  position,  have  an  assistant 
turn  the  cross  disk  so  that  one  of  its  diameters  coincides 
with  the  oblique  string.  Get  a  clear  after-image  of  it,  and 
look  at  the  fixation  point  on  that  string.  Again  the  bar  of 
the  cross  will  lie  exactty  upon  the  string,  thus  showing  that 
no  rotation  of  the  eye  about  the  line  of  sight  has  taken 
place.  The  same  would  be  true  for  any  other  direction  of 
motion  from  the  primary  position,  provided  the  movement 
were  not  of  extreme  extent.  There  is  then  a  set  of  lines, 
radiating  from  the  primary  fixation  point,  along  which  the 
eye  can  move,  so  as  to  bring  all  parts  of  the  same  line  suc- 
cessively on  the  same  part  of  the  retina.  Direct  examina- 
tion of  such  a  line  and  comparison  of  its  parts  is  easy. 

Restore  the  cross  disk  to  its  first  position,  incline  the 
head  forward  or  backward,  or  turn  it  to  right  or  left  before 
getting  the  after-image  (thus  bringing  the  eye  into  a  sec- 
ondary position),  and  repeat  the  experiments  just  made. 
Notice  that  the  bars  do  not  now  coincide  with  the  strings, 
showing  that  the  eyes  have  suffered  a  certain  amount  of 


124        LABORATORY  COURSE  IN  PSYCHOLOGY. 

rotation.  Such  a  rotation  appears  for  all  secondary  posi- 
tions (except  when  the  fixation  point  both  at  starting  and 
ending  lies  in  a  straight  line  passing  through  the  primary 
fixation  point) ,  but  the  extent  of  it  is  small  in  the  ordinary 
movements  of  the  eyes,  and  extreme  movements  are  usually 
avoided  by  simultaneous  movements  of  the  head. 

With  the  cross  on  the  disk  vertical  as  in  the  cut,  get  an 
after-image  and  fixate  the  mark  on  the  oblique  string.  In- 
stead of  being  rectangular  as  before,  the  after-image  cross 
now  appears  somewhat  distorted,  like  an  oblique  X.  The 
after-image  on  the  retina  of  course  remains  rectangular. 
The  distortion  of  the  image  on  the  wall  is  the  result  of  the 
interpretation  now  placed  upon  it  by  the  mind.  The  short 
string  cross  at  the  same  centre  is  known  to  be  rectangular, 
and  if  the  after-image  cross  fails  to  agree  with  it,  the  only 
harmonization  of  the  two  is  that  the  latter  is  not  really 
rectangular.  Oblique  crosses  in  such  a  position  in  previous 
experience  have  given  rise  to  rectangular  retinal  images 
so  often  that  this  interpretation  is  immediate,  and  seems 
wholly  a  matter  of  sensation. 

For  a  fuller  account  of  Listing's  Law  see  Appendix  I. 

Cf.  Helmholtz,  A,  Fr.  601-609  (462-469),  621  (479)  ff.,  702  (548)  ff.; 
Aubert,  A,  653  ff. ;  Wundt,  A,  3te.  Aufl.,  II.,  94  ff. ;  Hering,  J5,  248  ff. ; 
Le  Conte,  164-177. 

132.  Actual  Movements  of  the  Eyes.  Wundt-Lamansky 
Law.  Rapid  motions  of  the  eyes  when  they  move  freely 
and  do  not  follow  strongly  marked  lines  in  the  field  of  re- 
gard, are  not  executed  exactly  according  to  Listing's  Law, 
though  that  gives  correctly  the  end  positions  reached.  The 
axis  about  which  the  eye  turns  is  not  always  constant,  and 
the  paths  of  the  fixation  point  as  it  moves  in  the  field  of  re- 
gard are  therefore  not  all  straight.  This  is  easy  to  observe 
as  follows.  In  a  dark  room  turn  down  the  gas  till  it  burns 
in  a  very  small  flame.  Then  using  this  as  a  distant  point 


THE  MECHANISM  ^FjfjHB^YE.       J       125 

of  departure  in  the  primary  position,  look  suddenly  from  it 
to  other  points  of  fixation  in  various  directions  about  it,  and 
notice  the  shape  of  the  long  positive  after-images  that  result 
from  the' motion  of  the  image  of  the  flame  over  the  retina. 
These  will  probably  have  the 'shape  of  the  radii  in  the  left 
hand  figure  below,  the  vertical  and  horizontal  being  nearly 
straight,  and  the  oblique  curved.  These,  however,  do  not 
show  immediately  the  track  of  the  fixation  point.  The 
newest  part  of  the  after-image  is  that  next  the  light,  the 
oldest,  part  is  that  next  the  fixation  point  —  at  a  in  the 
diagram.  If  the  points  of  the  after-image  curve  are  now 
interpreted  in  the  order  of  time  (taking  the  oblique  curve  to 


the  right  and  upward,  for  example),  it  appears  that  the  eye 
at  first  moved  rather  rapidly  toward  the  right,  but  rather 
slowly  upward,  while  at  last  it  moved  rather  slowly  toward 
the  right  and  rapidly  upward.  Plotting  a  curve  in  accord- 
ance with  this  interpretation,  we  get  that  given  in  B,  which 
shows  the  true  track  of  the  fixation  point.  By  similar  plot- 
ting the  other  tracks  may  be  found. 

It  is  said  that  for  some  eyes  the  after-images,  though 
curved,  do  not  coincide  with  those  figured  in  A. 

Wundt,  B,  139  ff.,  201-202;  Bering,  A,  450-451;  Lamansky. 

133.  Convergent  Movements  of  the  Eyes.  The  laws  of 
Ex.  131  do  not  hold  for  convergent  motions  of  the  eyes. 


126        LABORATORY  COURSE  IN  PSYCHOLOGY. 

When  the  lines  of  sight  converge  in  the  primary  position, 
both  eyes  rotate  outward ;  as  the  lines  of  sight  are  elevated, 
the  convergence  remaining  the  same,  the  outward  rotation 
increases  ;  as  they  are  depressed,  the  rotation  diminishes 
and  finally  becomes  zero.  On' a  sheet  of  cardboard  draw  a 
series  of  equidistant  parallel  vertical  lines  one  or  two  inches 
apart  and  eight  or  ten  inches  long,  drawing  the  left  half  of 
the  group  in  black  ink,  the  right  half  in  red.  Cross  both 
sets  midway  from  top  to  bottom  by  a  horizontal  line,  red  in 
the  red  set,  and  black  in  the  black  set.  Fasten  the  card- 
board flat  upon  a  vertical  support,  and  arrange  the  head  rest 
in  front  of  it.  The  horizontal  line  of  the  diagram  should 
be  on  a  level  with  the  eyes. 

a.  If  the  operator  is  unable  to  control  the  degree  of  con- 
vergence voluntarily,  he  should  fasten  a  bit  of  wire  vertically 
between  his  eyes  and  the  diagram  in  such  a  way  that  it  can 
be  moved  to  and  from  the  eyes.     If  he  is  able  to  control  the 
convergence  voluntarily,  the  wire  is  unnecessary.     Bring 
the  head  into  position  and  converge  the  eyes,  giving  atten- 
tion to  the  diagram.     It  will  be  seen  that  the  red  and  black 
lines  are  not  quite  parallel  (or  do  not  quite  coincide),  and 
that  they  are  less  nearly  so  as  the  convergence  is  increased. 
The  red  lines  (seen  by  the  left  eye)  seem  to  incline  a  little 
toward  the  right,  and  the  black  lines  (seeji  by  the  right  eye) 
toward  the  left.     When  the  convergence  is  great,  the  hori- 
zontal lines  also  will  show  the  rotation.      This  apparent 
rotation  of  the  lines  is  not,  as  in  the  case  of  the  after-image, 
a  sign  that  the  corresponding  eye  has  rotated  in  the  same 
way,  but  that  it  has  rotated  in  the  opposite  way. 

b.  Repeat   this  with   the  head   much  inclined   forward 
(the  equivalent  of  elevating  the  eyes)  and  with  it  thrown 
far  back  (equivalent  of  depressing  the  eyes),  taking  care 
that  the  same  degree  of  convergence  is  maintained.     In  the 
first  case  the  apparent  rotation  of  the  lines  is  increased,  and 


THE  MECHANISM  OF  THE  EYE.  127 

in  the  second  decreased  to  zero,  or  even  transformed  into 
rotation  in  the  opposite  direction. 

Helmholtz,  A,  Fr.  609-610  (469-470);  Le  Conte,  177-191;  Hering, 
A,  496  ff. ;  Aubert,  A,  658  ff. 

134.  Involuntary  Movements  of  the  Eyes.  Lay  a  small 
scrap  of  red  paper  on  a  large  piece  of  blue.  Fixate  some 
point  on  the  edge  of  the  red.  After  a  few  seconds  of  steady 
fixation,  the  color  near  the  line  of  separation  will  be  seen 
to  brighten,  now  in  the  red  and  now  in  the  blue,  thus  be- 
traying the  small  unintentional  movements  of  the  eyes. 

Helmholtz,  A,  539,  Fr.  511  (389). 


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128        LABORATORY  COURSE  IN  PSYCHOLOGY. 

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THE  MECHANISM  OF  THE  EYE.  129 

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in  the  Eye,  American  Journal  of  Science,  2d  Ser.,  XXX., 
1860,  264.  Additional  observations  on  the  Circulation  in  the 
Eye,  ibid.,  385. 


130        LABORATORY  COURSE  IN  PSYCHOLOGY. 

SACHS:  Ueber  die  specifische  Lichtabsorption  des  gelben  Fleckes 
der  Netzhaut,  Pfluger's  Archiv,  L.,  1891,  574-586. 

SCHWAKZ:  Ueber  die  Wirkung  des  constanten  Stroms  auf  das 
normale  Auge,  Archivfilr  Psychiatric,  XXI.,  1890,  588-617. 

TREITEL:  Ueber  das  Yerhalten  der  normalen  Adaptation,  v.Graefe's 
Archiv,  XXXIII.,  1887,  ii.,  73-112. 

TSCHERNING:  Beitrage  zur  Dioptrik  des  Auges,  Zeitschrift  fur 
Psychologic,  III.,  1892,  429^92. 

TUMLTRZ:  Ueber  ein  einfacbes  Verfahren,  die  Farbenzerstreuung 
des  Auges  direkt  zu  sehen,  Pfluger's  Archiv,  XL.,  1887,  394. 

UHTHOFF:  Ueber  die  kleinsten  wahrnehmbaren  Gesichtswinkel  in 
den  verscbiedenen  Teilen  des  Spektrums,  Zeitschrift  fur  Psy- 
chologic, I.,  1890,  155-160.  Contains  bibliographical  notices 
on  minimum  visibile. 

WOLF  :  Ueber  die  Farbenzerstreuung  im  Auge,  Wicdemanri's  An- 
nalen,  XXXIII.,  1888,  548-554. 

WUNDT  :    A.  Work  cited  in  bibliography  of  Chapter  I. 

B.  Beitrage  zur  Theorie  der  Sinneswahrnehmung,  Leipzig,  1862. 

ZEHFUSS:  Ueber  Bewegungsnachbilder,  Wiedemanri1  s  Annalen,  IX., 

1880,  672-676. 
The  works  of  Helmholtz  and  Aubert  mentioned  above  contain 

full  bibliographies  for  the  earlier  literature  of  all  the  subjects  con- 
sidered in  this  and  the  next  two  chapters. 


SENSATIONS   OF  LIGHT  AND   COLOR.  131 


CHAPTER  VI. 
Sensations  of  Light  and  Color. 

THE  aim  of  the  following  experiments  is  not  to  settle 
conflicting  color  theories,  but  rather  to  present  the  most  im- 
portant experimental  facts  which  all  color  theories  nmst 
take  into  account.1  Authoritative  statements  of  theories 
may  be  found  as  follows :  Young-Helmholtz  theory  ;  Helm- 
holtz,  A,  344-350,  Fr.  380-387,  424-425,  484  (290-294, 
320-321,  367)  ;  B,  249-256.  Bering's  theory ;  Bering,  A, 
70-141 ;  M,  76-79.  Hering  has  not  yet  made  a  general  state- 
ment of  his  theory  in  its  later  developments,  and  his  present 
views  must  be  gathered  in  more  or  less  fragmentary  con- 
dition from  his  numerous  special  articles.  The  theories  of 
Helmholtz  and  Hering  are  the  most  prominent  of  current 
theories  ;  and  something  on  them,  especially  on  the  first, 
will  be  found  in  the  physiologies  generally,  and  in  some 
works  on  color  in  the  arts.  Of  other  theories  there  are  a 
considerable  number ;  see,  for  some  of  them,  von  Kries ; 
Wundt,  A,  and  B;  Bonders,  A  and  B\  Christine  Ladd 
Franklin,  A  and  B ;  Ebbinghaus,  A. 

Most  color  theories  attempt  to  simplify  the  multiplicity 
of  ordinary  color  sensations  by  considering  them  as  com- 
pounds of  a  small  number  of  simple  or  primary  sensations. 
The  number  of  primary  colors  is  different  in  different 
theories ;  red,  green,  and  violet  (or  blue)  are  selected  by 


1  For  concise  statements  of  these  facts,  see  Wundt,  A,  3te  Aufl.,  I.,  487,  501, 
4te  Aufl.,  I.,  529;  and  Christine  Ladd  Franklin,  A. 


132         LABORATORY  COURSE  IN  PSYCHOLOGY. 

the  supporters  of  the  Young-Helmholtz  theory  ;  red,  green, 
yellow,  and  blue  by  Hering,  Mach,  and  others;  while 
Wundt  is  indisposed  to  make  any  particular  colors  more 
original  for  sensation  than  the  rest.  The  selection  has 
generally  been  dictated  by  considerations  of  physics,  or  the 
results  of  introspective  analysis  of  the  sensations ;  but 
efforts  have  lately  been  made  to  settle  the  question  by 
careful  examination  of  the  color-blind,  and  by  calcula- 
tions based  upon  careful  experiments.  On  the  first,  see 
the  literature  on  color-blindness  below ;  on  the  second,  see 
Helmholtz,  A,  456  if.,  D,  and  Konig  und  Dieterici,  A. 
White  is  unquestionably  a  sensation,  and  Helmholtz  and 
Hering  agree  in  holding  the  same  with  reference  to  black ; 
though  Fick  and  some  others  disagree,  regarding  it  rather 
as  the  absence  of  sensation. 

A  given  color  sensation  may  be  changed  in  three  ways : 
in  color-tone,  in  saturation,  and  in  intensity,  or,  to  use 
Maxwell's  terms,  in  hue,  tint,  and  shade.  Changes  in  color- 
tone  are  such  as  are  experienced  when  the  eye  runs  through 
the  successive  colors  of  the  spectrum.  Changes  in  satura- 
tion are  such  as  are  produced  by  the  addition  or  subtrac- 
tion of  white ;  when  much  white  light  is  added,  the  color 
is  a  little  saturated.  Changes  in  intensity  are  changes  in 
the  brightness  of  the  color.  Changes  in  saturation  and  in 
intensity,  if  excessive,  involve  some  change  of  color-tone 
also.  Hering' s  theory  does  not  admit  changes  in  the  inten- 
sity of  light  and  color  sensations  in  any  ordinary  sense  of 
the  word.  Colors  that  by  others  are  said  to  be  of  low  in- 
tensity are  regarded  by  Hering  and  his  school  as  mixed 
with  a  large  proportion  of  black ;  similarly  those  of  high 
intensity  are  mixed  with  much  white.  In  Hering's  theory 
the  possible  changes  are  then  reduced  to  two ;  changes  in 
color-tone  and  in  saturation"  the  latter  including  admixtures 
of  both  white  and  black  (Hillebrand;  Hering,  A,  51  if.). 


SENSATIONS   OF  LIGHT  AND  COLOR.  133 

In  this  group  of  experiments  it  has  seemed  best  to  follow 
the  better  known  terminology,  though  Hering's  conception 
of  the  matter  ought  not  to  be  disregarded. 

LIGHT  AND  COLOR  IN  GENERAL. 

135.  Color-Blindness,  Holmgren's  Method,  a.  Spread 
the  worsteds  on  a  white  cloth  in  good  daylight.  Pick  out  a 
pale  green  (i.  e.,  a  little  saturated  green)  that  leans  neither 
toward  the  blue  nor  the  yellow  ;  lay  it  by  itself  and  require 
the  person  under  examination  to  pick  out  and  lay  beside  it 
all  other  skeins  that  are  colored  like  it,  not  confining  him- 
self, however,  to  exact  matches,  but  taking  somewhat  darker 
and  lighter  shades  also,  so  long  as  the  difference  is  only  in 
brightness  and  not  in  color-tone.  Do  not  tell  him  to  pick 
out  "the  greens"  nor  require  him  to  use  or  understand 
color  words  in  any  way ;  simply  require  the  sorting.  If 
he  makes  errors,  putting  grays,  light  browns,  salmons,  or 
straws l  with  the  green,  he  is  color-blind ;  if  he  hesitates 
over  the  erroneous  colors  and  has  considerable  difficulty,  his 
color- vision  is  probably  defective,  but  in  a  less  degree. 

b.  If  the  experimentee  makes  errors,  try  him  further  to 
discover  whether  he  is  "  red-blind "  or  "  green-blind  "  by 
asking  him  to  select  the  colors,  including  darker  and  lighter 
shades,  that  resemble  a  purple  (magenta)  skein.  If  he  is 
red-blind,  he  will  err  by  selecting  blues  or  violets,  or  both ; 
if  he  is  green-blind,  he  will  select  green  or  gray,  or  both, 
and  if  he  chooses  any  blues  and  violets,  they  will  be  the 
brightest  shades.  If  he  makes  no  errors  in  this  case,  after 
having  made  them  in  the  previous  case,  his  color-blindness  is 
incomplete.  Violet-blindness  is  rare.  See  also  ExI  141  b. 

Complete  certainty  in  the  use   of  even  such   a   simple 

l  It  is  difficult  to  give  the  tints  accurately  in  words.  The  experimenter  should 
consult  the  colored  charts  given  in  the  works  of  Jeffries  mentioned  in  the  bibli- 
ography, and  in  Rayleigh,  B. 


134         LABORATORY  COURSE  IN  PSYCHOLOGY. 

method  as  this  is  not  to  be  expected  without  a  full  study  of 
it  and  experience  in  its  application.  Helmholtz,  Hering, 
Konig,  Kirschmann,  and  others  give  exact  methods  for 
determining  the  particular  colors  that  are  lacking  in  the 
vision  of  the  color-blind. 

On  color-blindness  and  methods  of  testing  for  it,  see  Helmholtz, 
A,  357-372,  456-462;  Fr.  388-399,  (294-300,  847-848);  Holmgren; 
Jeffries,  A  and  B;  Rayleigh,  A  and  B\  Bering,  H,  I,  Ny  Hess,  B; 
Abney,  A-  Abney  and  Festing;  Konig,  B  and  C;  Brodhun,  A  and 
J3;  Konig  und  Brodhun;  Konig  und  Dieterici,  A\  Schuster;  Preyer; 
Donders,  (7;  Kirschmann,  A'  Pole. 

136.  Vision  with  Peripheral  Portions  of  the  Eetina: 
Perception  of  Light.  A  very  faint  light  often  appears 
brighter  when  its  image  lies  not  in  the  fovea,  but  a  few 
degrees  away  from  it.  If  no  increase  of  brightness  is  ob- 
served, it  is  at  least  difficult  to  trace  any  decrease  in  bright- 
ness till  the  image  is  many  degrees  from  the  fovea.  This 
experiment  is  most  easily  made  at  night  with  faint  stars. 
In  the  laboratory  it  may  be  made  with  the  dark  box.  On 
the  rear  wall  of  the  box  place  in  a  horizontal  line  three 
bits  of  white  paper  of  equal  size,  at  such  distances  that  the 
line  of  sight  moves  through  an  angle  of  ten  degrees  in  turn- 
ing from  the  middle  one  to  either  of  the  outer  ones.  Make 
a  pin-hole  above  and  below  the  middle  piece,  distant  from  it 
about  an  inch,  and  cover  the  holes  on  the  outside  with 
paper  till  the  holes  are  barely  visible  after  the  eye  has  been 
some  time  adapted.  These  bright  points  serve  to  steady 
the  eye.  The  eye  should  not,  however,  'be  directly  fixed 
upon  them,  but  at  a  point  midway  between  them.  Reduce 
the  illumination  of  the  box  to  a  minimum  (e.  g.,  to  the 
amount  of  light  that  would  enter  through  a  pin-hole  cov- 
ered with  one  or  more  pieces  of  porcelain  or  translucent 
cards),  wrap  the  head  and  the  end  of  the  box  in  an  opaque 
cloth,  and  allow  the  eyes  to  become  adapted  to  the  darkness, 


SENSATIONS   OF  LIGHT  AND  COLOR.  135 

looking  from  time  to  time  for  the  shimmer  of  the  papers  at 
the  back  of  the  box.  Full  adaptation  requires  a  long  time, 
but  fifteen  minutes  is  sufficient  in  this  case.  By  degrees,  if 
the  illumination  is  of  the  right  intensity,  the  papers  will  be 
seen  very  faintly.  If  the  eye  is  turned  directly  towards 
one  of  them,  it  often  disappears  in  the  retinal  light  while 
the  others  brighten.  Fixate  each  of  them  successively,  and 
compare  its  brightness  with  the  others ;  fixate  also  other 
points  in  the  field  so  as  to  bring  the  images  upon  different 
quadrants  of  the  retina.  Close  the  eyes  from  time  to  time 
to  renew  the  adaptation,  and  avoid  observations  when  the 
retinal  light  is  strongly  concentrated  in  the  centre  of  the 
field. 

On  the  results  of  such  experiments  as  this,  and  on  the  explana- 
tion of  the  phenomenon  observed,  experimenters  are  somewhat  at 
variance,  but  see  Helmholtz,  A,  268;  Aubert,  A,  495,  B,  89  ff.;  A. 
E.  Fick,  B\  Kirschmann,  B;  Treitel,  and  the  literature  cited  by 
them. 

137.  Vision  with  Peripheral  Portions  of  the  Retina: 
Perception  of  Color.  The  distribution  of  the  sensibility  of 
the  retina  for  color  is  unlike  that  for  light.  At  the  very 
centre  the  pigment  of  the  yellow  spot  itself  interferes  some- 
what with  the  correct  perception  of  mixed  colors  (see  Ex. 
115).  In  a  zone  immediately  surrounding  this  all  colors 
can  be  recognized.  Outside  of  this  again  is  a  second  zone 
in  which  blue  and  yellow  alone  can  be  distinguished,  and 
at  the  outermost  parts  not  even  these,  all  colors  appearing 
black,  white,  or  gray.  The  zones  are  not  sharply  bounded, 
but  blend  into  one  another,  their  limits  depending  on  the 
intensity  and  area  of  the  colors  used.  The  fixing  of  the 
boundaries  of  the  zones  of  sensibility  is  known  as  perimetry 
or  campimetry. 

a.  With  the  apparatus  at  hand,  find  at  what  angles  from 
the  centre  of  vision  on  the  vertical  and  horizontal  meridians 


136         LABORATORY  COURSE  IN  PSYCHOLOGY. 

of  the  eye  the  four  principal  colors,  red,  yellow,  green,  and 
blue,  can  be  recognized;  try  white  also.  Keep  the  eye 
steadily  fixed  on  the  fixation  mark  of  the  instrument,  and 
have  an  assistant  slide  the  color  (say  a  bit  of  colored  paper 
5  mm.  square  pasted  near  the  end  of  a  strip  of  black  card- 
board an  inch  wide)  slowly  into  the  field  from  the  outside. 
It  will  be  well  to  move  the  paper  slowly  to  and  fro  at  right 
angles  to  the  meridian  on  which  the  test  is  made,  so  as  to 
avoid  retinal  fatigue.  Take  a  record  of  the  point  at  which 
the  color  can  first  be  recognized  with  certainty.  Repeat 
several  times  and  average  the  results.  The  size  of  the 
colored  spot  shown  should  be  constant  for  the  different 
colors,  and  the  background  (preferably  black)  against  which 
the  colors  are  seen  should  remain  the  same  in  all  the 
experiments. 

b.  Repeat  the  tests  with  colored  squares  20  mm.  on  the 
side,  and  notice  the  earlier  recognition  of  their  color  as  they 
approach  from  the  periphery. 

c.  Try  bringing  slowly  into  the  field  (best  from  the  nasal 
side)  bits  of  paper  of  various  colors,  especially  violet,  pur- 

.  pie,  orange,  greenish  yellow,  and  greenish  blue ;  or  better, 
hold  the  bit  of  paper  somewhat  on  the  nasal  side  of  the 
field  and  turn  the  eye  slowly  toward  it,  beginning  at  a  con- 
siderable angle  from  it.  If  the  paper  is  held  before  a  back- 
ground containing  a  line  along  which  the  eye  can  approach 
the  paper,  the  eye  will  be  assisted  in  making  the  approach 
gradual ;  the  apparatus  used  in  Ex.  113  b  can  easily  be 
adapted  for  this  purpose.  Observe  that  on  the  outer  parts 
of  the  retina  these  colors  first  get  their  yellow  or  blue  com- 
ponents, and  only  later  the  red  or  green.  If  the  range  of 
choice  is  sufficiently  large,  it  may  be  possible  to  find  a  red 
(inclined  toward  red-purple)  and  a  green  (inclined  toward 
the  blue),  which,  like  pure  blue  and  yellow,  change  only  in 
saturation  and  not  at  all  in  color-tone  as  they  move  inward 


SENSATIONS   OF  LIGHT  AND  COLOR.  137 

toward  the  centre  of  the  field.     These  four  colors  are  the 
Urfarben  or  primary  colors  of  Hering. 

Helmholtz,  A,  372-374,  Fr.  399-400;    Hess,  A;    Hering,  <2,  £; 

A.  Fick,  A,^B,  206  ff. ;  A.  E.  Fick,  J5,  479  ff. ;  Aubert,  A,  539-546,  B, 
116  ff. ;  Kirschmann,  C. 

138.  Changes  in  Color-Tone.     In  the  spectrum,  change 
of  wave-length,  if  not  too  small,  is  accompanied  by  change 
of  color-tone.     The  change  is  most   rapid   in  the  yellow- 
green  and  blue-green  regions  of  the  spectrum,  less  rapid 
toward  the  ends,  and  at  the  extreme  ends  the  only  changes 
are  those  in  brightness.     With  the  spectroscope  and  day- 
light find  the  characteristic  Fraunhofer  lines  D,  E,  F,  G,  and 
ff.     The  D  line  lies  in  the  golden  yellow,  F  in  the  greenish 
blue,  and  H  at  the  end  of  the  violet.     Between  D  and  F  the 
wave-length  changes  from  589.2  to  486.1  pp.  (from  5.092  X 
1014to  6.172  X  1C14  vibrations  per  second),  and  the  color  runs 
through  yellow  and  green  to  blue,  while  from  F  to  ff  with 
the  nearly  proportional  change  in  wave-length  from  486.1 
to  393.3  fjLft,  (from  6.172  X  1014  to  7.628  X  1014  vibrations 
per  second)  the  change  is  only  from  greenish  blue  to  violet. 
Notice  the  region  from  near  the  line  G  to  the  end  of  the 
spectrum  which  shows  little  change  in  color-tone  and  a  simi- 
lar region  of  uniform  color-tone  at  the  red  end.     Notice 
also  the  tendency  of  the  succession  of  spectral  colors  to  return 
upon  itself,  shown  in  the  resemblance  of  the  violet  and  red. 

Helmholtz,^,  289,320,  Fr.  319  (237);  Wundt,  3te  Aufl.,  L,  449  f., 
4te  Aufl.,  I.,  485  f. ;  A.  Fick,  B  ;  Aubert,  A,  530  f.  On  just  observ- 
able changes  in  color-tone,  see  B.  O.  Peirce,  Jr.,  Konigund  Dieterici, 

B,  Brodhun,  -4,  and  the  literature  there  cited. 

139.  Changes  in  Saturation.     These  are  easily  shown  on 
the  color-mixer.     Make  a  succession  of  mixtures  of  red  and 
white,   beginning   with   a   proportion   of    white   that   just 
changes  the  red,  and  increase  the  proportion  till  no  effect 
of  red  remains.     At  first  use  a  small  disk  of  red  laid  on 


138         LABORATOET  COURSE  IN  PSYCHOLOGY. 

over  the  larger  disks  as  a  sample  with  which  to  compare  the 
mixtures.  Toward  the  end  of  the  experiment  exchange  the 
red  for  a  small  white  disk.  Notice  the  changes  of  color- 
tone  that  are  to  be  observed,  especially  when  the  amount  of 
color  is  small.  Try  similarly  with  the  other  chief  colors. 
According  to  Rood,  who  worked  with  the  color-mixer,  yellow- 
green  and  violet  are  unchanged;  Helmholtz's  results  with 
spectral  colors  are  somewhat  different. 

Changes  in  saturation  can  also  be  made  by  adding  gray 
of  any  shade  instead  of  white.  The  whole  range  of  mix- 
tures can  be  shown  on  a  single  disk,  like  that  in  Ex.  141,  by 
painting  the  star  upon  a  white  or  gray  ground,  or  by  past- 
ing a  star  of  colored  paper  on  such  a  ground.  With  white, 
however,  the  rays  of  the  star  must  be  given  a  leaf  shape,  or 
the  color  will  fall  off  too  rapidly  from  the  centre. 

Helmholtz,  A,  322,  470-471,  Fr.  369  (281);  Aubert,  A,  531-532; 
Kood,  A,  39-40,  194-201;  Nichols,  A. 

140.  Changes  in  Intensity :  Black  and  White.  Black  and 
white  are  the  extremes  of  intensity  in  the  series  of  grays. 
The  ordinary  black  and  white  of  conversation  are,  however, 
considerably  short  of  these  extremes. 

a.  Compare  a  bit  of  black  velvet  or  of  black  cardboard 
with  a  still  deeper  black  by  holding  it  in  front  of  the  open- 
ing in  the  dark  box.     Compare,  also,  ordinary  white  paper 
in  diffused  light  with  the  same  in  direct  sunlight,  or  with  a 
brightly  illuminated  white  cloud. 

b.  Just  observable  differences  with  medium  intensities. 
Prepare  a  disk  like  that  shown  in  the  accompanying  cut  by 
drawing  along  a  radius  of  a  white  disk  a  succession  of  short 
black  lines  of  equal  breadth.     Let  the  breadth  of  the  line 
correspond  to  about  one  degree  on  the  edge  of  the  disk. 
Since  the  breadth  of  the  line  is  everywhere  the  same,  it 
will  occupy  a  relatively  greater  angle  as  it  nears  the  centre. 


SENSATIONS   OF  LIGHT  AND   COLOR.  139 

When  the  disk  is  set  in  rapid  rotation,  each  short  line 
will  give  a  faint  gray  ring,  those  at  the  outer  edge  being 
very  faint,  those  nearer  the 
centre,  darker.  Find  which 
is  the  faintest  ring  that  can 
be  seen,  and  calculate  the 
proportions  of  black  and 
white  in  it.1  The  ratio  of 
black  to  white  measures 
approximately  the  just  ob- 
servable decrease  in  in- 
tensity below  the  general 
brightness  of  the-  disk. 
The  results  of  Helmholtz 
and  Aubert  are  respec- 
tively :  Helmholtz,  1 : 117  to  1 : 167,  Aubert,  1 : 102  to  1 : 186, 
the  differences  depending  on  the  intensity  of  the  general 
illumination  of  the  disk.  Some  wandering  of  the  eyes  is 
helpful,  but  too  rapid  motions  which  tend  to  break  up  the 
even  gray  of  the  rings  must  be  avoided.  It  is  absolutely 
essential  that  the  rotation  be  very  rapid  and  perfectly  free 
from  vibration  —  so  rapid  that  with  moderate  motions  of 
the  eyes  the  uniform  gray  of  the  rings  is  not  disturbed.  If 
great  rapidity  is  impossible,  replace  the  single  black  line 
by  two  of  proportionately  less  breadth  on  opposite  sides  of 
the  disk,  or  by  four  at  90°. 

c.   With  these  very  faint  rings  a  disappearance  and  reap- 
pearance is  to  be  observed  somewhat  like  that  found  for 

1  The  formula  for  the  amount  of  black,  assuming  that  the  radial  line  is  abso* 
lutely  black,  and  taking  some  arbitrary  point,  e.g.,  the  middle,  for  calculation,  is  of 

course — - — .  where  b  is  the  breadth  of  the  radial  line,  and  r  the  distance  of  the 

27r  r 

chosen  point  from  the  centre  of  the  disk.  The  black  of  the  lines  is  not  quite  abso- 
lute, even  when  the  blackest  black  paint  is  used.  The  differences  in  sensation 
are  therefore  smaller  than  those  shown  by  the  calculation. 


140         LABORATORY  COURSE  IN  PSYCHOLOGY. 

just  audible  sounds  in  Ex.  61  b.  The  observation  is  most 
conveniently  made,  according  to  Pace,  on  a  disk  of  the  fol- 
lowing dimensions  :  diameter  of  disk,  20  cm.,  width  of  radial 
line,  5  mm.,  length  of  the  short  lines,  5  mm.,  spaces  between 
the  short  lines,  8  mm.,  distance  of  innermost  short  line  from 
the  centre  of  the  disk,  17  mm. 

Helmholtz,  A,  384-393;  Fr.  411-419  (310-316);  Aubert,  A,  487- 
492 ;  on  c,  Pace.  For  references  on  the  just  observable  difference  of 
intensity  with  different  standard  intensities,  see  the  chapter  on 
Weber's  Law  below. 

141.  Changes  in  Intensity  :  Colors.  At  their  maximum 
intensity  all  colors  tend  toward  white  or  yellowish  white. 
Red,  however,  hardly  gets  beyond  the  yellow ;  green  be- 
comes first  yellow,  then  white,  while  blue  and  violet  easily 
reach  it.  At  their  minimum  intensity  all  colors  appear 
gray  or  black. 

a.  The  maximum  intensity  may  be  observed  with  spec- 
tral colors,  though  not  entirely  homogeneous  ones,  with  a 
prism  placed  in  the  sunlight  so  that  it  throws  an  extended 
spectrum  on  the  wall.     Hold  a  card,  pierced  with  a  pin-hole, 
before  the  eye,  and  bring  the  eye  successively  into  the  dif- 
ferent colors,  looking  meanwhile  at  the  prism.     Something 
of  the  same  kind  may  be  seen  by  looking  through  pieces  of 
colored  glass  at  the  disk  of  the  sun  behind  a  cloud  (in  which 
case  the  portions  of  the  cloud  seen  at  the  sides  of  the  glass 
afford  a  means  of  comparison),  or  at  the  image  of  the  sun 
reflected  from  an  unsilvered  glass  plate,  or  by  concentrating 
light  from  colored  glass  on  white  paper  with  a  convex  lens. 

b.  The  minimum  intensity  with  spectral  colors  may  be 
observed  with  a  spectroscope.     Adjust  the  instrument   so 
that  the  chief  Fraunhofer  lines  can  be  seen,  and  then  place, 
as  a  source  of  light,  at  a  little  distance  from  the  slit  of  the 
instrument,  a  screen  covered  with  dark  gray  paper  or  black 
velvet.     Though  no  color  remains,  a  little  light  can  be  made 


SENSATIONS   OF  LIGHT  AND  COLOR.  141 

out  —  brightest  in  the  region  before  occupied  by  the  green. 
The  observer  must  envelop  his  head  and  the  ocular  of  the 
instrument  in  an  opaque  cloth,  and  allow  time  for  the  adap- 
tation of  his  eye.  This  colorless  spectrum  probably  repre- 
sents what  is  seen  by  a  totally  color-blind  eye. 

Von  Bezold,  with  whom  this  experiment  originates,  ob- 
served with  gradually  decreasing  intensity  a  falling  out  of 
the  yellows  and  blues  before  the  final  stage  of  colorlessness 
was  reached.  Konig  doubts  whether  the  red  ever  loses 
its  color  entirely. 

With  pigment  colors  a  convenient  way  is  to  paste  equal 
squares  of  colored  papers  upon  a  piece  of  cardboard,  and 
then  to  place  the  whole  in 
the  dark  box,  and  gradually 
reduce  the  illumination,  or 
starting  with  the  illumina- 
tion at  zero,  gradually  in- 
crease it.  Try  with  both 
black  and  white  cardboard 
as  background.  For  dem- 
onstrational  purposes  a  disk 
like  that  in  the  accompany- 
ing cut  (in  which  the  shaded 
part  stands  for  color,  and 
the  solid  black  for  black)  may  be  used  and  the  whole  series 
of  intensities  shown  at  once.1 

Helmholtz,  A,  402-444  ;  A.  Fick,  B,  200-202  ;  Aubert,  ^1,532-536  ; 
Rood,  A,  181-194  ;  C.  S.  Peirce.  On  cr,  Helmholtz,  A,  284-285, 
465-466,  Fr.  315  (234);  Brodhun,  B.  On  6,  Helmholtz,  A,  469,  471- 


1  Since  the  black  of  the  disk  is  really  a  very  dark  gray,  and  would  thus  make  a 
change  in  saturation,  this  is  not  an  absolutely  pure  experiment,  but  is  sufficiently 
exact  for  showing  the  general  effect  of  darkening.  If  a  practically  perfect  black 
is  desired,  it  may  be  had,  following  Rood,  by  making  the  colored  star  rotate 
before  an  opening  into  a  dark  room  or  a  suitable  dark  box. 


142         LABORATORY  COURSE  IN  PSYCHOLOGY. 

472;  von  Bezold,  A]  Ebert;  Abney  and  Testing;  Konig,  A,  354  ff., 
where  other  literature  is  cited. 

For  measurements  of  the  just  observable  difference  of  intensity 
for  different  colors,  see  Helmholtz,  -4,  402-415;  Aubert,  A,  531; 
A.  Fick,  A,  177;  and  the  references  given  by  them. 

142.  Purkinje's   Phenomenon.     In  a  light  of  moderate 
brightness  choose  a  bit  of  red  paper  and  a  bit  of  blue  paper 
that  are  of  about  equal  intensity  and  saturation,  carry  both 
into  full  sunlight  and  notice  which  appears  brightest ;  carry 
both  into  a  darkened  room,  or  place  them  in  the  dark  box 
and  compare  them  again.     If  a  dark  room  or  box  is  not  at 
hand,  observe  them  through  a  fine  pin-hole  in  a  card,  or 
even  with  nearly  closed  eyes. 

Helmholtz,  A,  428-430,  443-444,  Fr.  420-425  (317-321);  Hillebrand; 
Konig,  A-,  Charpentier,  A,  227  ff.,  335  ff.;  Hood,  A,  189  ff. 

143.  Size  of  the  Colored  Field.     When  the  retinal  area 
stimulated  is  very  small,  colored  surfaces  appear  colorless, 
with  ordinary  intensities  of  illumination.     When  somewhat 
larger  they  may  appear  colored,  but  not  necessarily  in  their 
true  color-tone.     The  background  against  which  they  are 
placed  is  also  important. 

a.  On  pieces  of  black  and  white  cardboard,  paste  small 
squares  of  several  kinds  of  colored  paper,  one  series  5  mm. 
square,  one  2  mm.  square,  and  one  1  mm.  square.     Walk 
backward  from  them  and  notice  their  loss  of  color.     Ob- 
serve also  the  changes  in  color-tone. 

b.  A  number  of  retinal  impressions,  even  when  not  con- 
tiguous, are  mutually  supportive  in  color  effect.     This  is 
conveniently  shown  in  the  indirect  field.      In  a  two-inch 
square  of  black  cardboard,  punch  sixteen  holes  arranged  in 
the  form  of  a  square,  four  rows  of  four  holes  each.     The 
holes  should  be  an  eighth  or  three-sixteenths  of  an  inch  in 
diameter,  and  be  separated  by  spaces  of  the  same  extent. 
Paste  upon  the  back  of  the  square  a  piece  of  red  paper  of 


SENSATIONS   OF  LIGHT  AND  COLOR.  143 

sufficient  size  to  cover  the  holes,  thus  making  of  them  six- 
teen little  red  circles.  Prepare  also  another  piece  of  black 
cardboard  of  such  shape  that  it  may  be  laid  over  the  square 
and  cover  all  the  holes  except  one  of  the  corner  ones,  and 
again  when  necessary  may  easily  be  removed. 

With  the  apparatus  used  in  Ex.  137,  find  the  point  on  the 
nasal  half  of  the  retinal  horizon  where  the  single  red  circle 
can  just  no  longer  be  seen  in  its  true  color.  In  making  this 
determination,  the  square  should  be  so  held  that  the  diag- 
onal to  which  the  uncovered  circle  belongs  is  horizontal. 
When  the  point  has  been  found,  uncover  the  remaining 
fifteen  circles  (all  farther  toward  the  periphery),  and  notice 
that  the  color  of  the  group  can  be  seen  distinctly.  Fatigue 
in  fixing  the  limit  at  which  the  circle  can  be  seen  should  be 
avoided. 

On  a,  Helmholtz,  A,  374-375,  Fr.  399-400  (300);  Aubert,  A,  536- 
539;  Bering,  R,  18.  On  6,  A.  E.  Tick,  A  and  B  (especially  451-452). 

144.  Duration  of  Illumination.  Fechner's  Colors.  The 
retinal  inertia  is  different  for  different  colors.  In  the  ex- 
periments on  after-images  (Ex.  125  eZ),  it  was  observed  that 
the  after-image  of  a  white  surface  faded  away  through  a 
succession  of  colors ;  a  succession  of  colors  appears  also  to 
result  from  a  very  brief  vision  of  a  white  surface.  This 
can  be  seen  upon  almost  any  slowly  rotating  disk  of  black 
and  white ;  those  used  in  Exs.  128  b  and  145  c  show  the 
colors  well,  and  that  in  Ex.  145  a  shows  something  of  the 
dependence  of  particular  colors  upon  particular  rates  of 
recurrence.  Rotate  any  of  these  disks  with  less  rapidity 
than  that  required  for  a  uniform  gray,  and,  keeping  the 
eyes  steadily  fixed  upon  some  point  of  its  surface,  notice 
both  the  advancing  and  the  retreating  edges  of  the  white 
portions  of  the  disk.  The  colors  may  not  appear  instantly, 
but  are  not  difficult  to  get  with  attentive  gazing. 

Very  striking  and  beautiful  effects  can  be  obtained  by 


144         LABORATORY  COURSE  IN  PSYCHOLOGY. 

substituting  for  the  black  and  white  disk  a  black  one  from 
which  narrow  sectors  have  been  removed.  This  pierced 
disk  is  rotated  before  a  brightly  lighted  background,  e.  g., 
a  sheet  of  white  cardboard  in  full  sunlight,  a  bright  cloud, 
or  the  clear  sky,  and  the  eye  is  brought  very  close  to  the 
disk. 

Helmholtz,  A,  530-533,  Fr.  500-504  (380-383) ;  Fechner,  A ;  Briicke ; 
Exner;  Aubert,^!,  560;  Kood,  A,  92  ff.,  B\  Nichols,  jB;  Charpentier, 
B  and  C. 

145.  Rate  of  Rotation  Required  for  a  Uniform  Blending 
of  Black  and  White.  All  blending  of  colors  by  rotation 
depends  on  the  phenomenon  of  positive  after-images  (Ex. 
125).  A  disturbance  once  set  up  in  the  retina  does  not  at 
once  subside,  but  continues  an  instant  after  the  removal  of 
the  stimulus.  If  stimuli  follow  in  sufficiently  rapid  succes- 
sion the  disturbances  fuse,  and  the  result  is  the  same  as  if 
the  stimuli  had  been  mixed  before  reaching  the  retina.  A 
rough  determination  of  the  rate  required  for  uniform  blend- 
ing may  be  made  with  the  color-mixer  and  a  metronome. 
a.  Place  the  color-mixer  in  such  a  position  that  the  disk 
(like  that  in  the  margin)  shall 
be  illuminated  by  diffused 
daylight  only.  Turn  the 
driving-wheel  slowly  and 
ascertain,  by  counting,  how 
many  turns  of  the  disk 
correspond  to  one  turn  of 
the  driving-wheel.  Start 
the  metronome,  and  turn 
the  driving-wheel  in  time 
to  its  beats,  making  a  turn 
every  one.  two  or  four  beats. 
Notice  which  of  the  rings,  if  any,  is  just  blended  into  a 
uniform  gray.  If  none  is  just  blended,  change  the  rate  of 


SENSATIONS   OF  LIGHT  AND   COLOR.  145 

the  metronome  a  little,  and  repeat  the  trial  till  such  a  one 
is  found.  From  the  rate  of  the  metronome,  the  number  of 
turns  of  the  driving-wheel,  and  the  number  of  white  sectors 
in  the  just  blended  ring,  find  the  number  of  stimuli  per 
second  required.  The  experiment  is  easier  when  two 
observers  work  together,  one  giving  his  attention  to  the 
regular  driving  of  the  color-mixer,  and  the  other  to  watch- 
ing the  disk.  The  driving-belt  of  the  instrument  must 
be  tight  enough  not  to  slip,  and  the  metronome  should 
be  kept  well  wound  up.  Its  scale  should  also  be  verified 
by  counting  with  a  watch.  The  observer  must  of  course 
avoid  eye  motions  which  break  up  the  uniformity  of  the 
gray. 

b.  Eepeat  the  determination  with  the  disk  in  direct  sun- 
light ;  also  in  a  partially  darkened  room  or  at  twilight. 

c.  A  disk  like  that  in  the  margin   shows   mixtures  of 
several  different  proportions  of 

black  and  white  at  once.     If 

such  a  disk  is  brought  slowly 

to  the  rate  just  neces- 

sary to  give  a  uniform 

gray  at  the   centre,  a 

little  flickering  can  still 

be  traced  in  the  outer 

rings.     Care  should  be 

taken  not  to  fixate  the 

middle  of  the  disk  ex- 

clusively, for  with  mod- 

erate illumination  the  per- 

iphery of  the  retina  requires  a 

little  greater  speed  for  uniform 

blending  than  the  centre.     Helmholtz  states  that  little  dif- 

ference is  to  be  observed  in  the  rate  at  which  the  flickering 

ceases  with  the  somewhat  similar  disk  shown  at  the  left  in 


OF  THE 

UNIVERSITY  )) 

JJ 


146         LABORATORY  COURSE  IN  PSYCHOLOGY. 

Ex.    152  d,  but  with  that  given  here,  it  is  believed  that 
careful  observation  will  not  fail  to  show  a  difference. 

Helmholtz,  A,  488  ff.,  Fr.  453  (344)  ff.;  Aubert,  A,  517;  A.  Fick, 
J3,  211-222;  Nichols,  J5;  Bellarminow  and  the  literature  cited  by 
him. 

146.  The  Talbot-Plateau  Law.     This  law  may  be  stated 
as  follows :  When  once  the  rate  of  rotation  is  sufficient  to 
give  a  uniform  sensation,  the  color  and  brightness  of  any 
given  concentric  ring  of  the  disk  are  the  same  that  they 
would  be  if  all  the  light  reflected  from  it  were  evenly  dis- 
tributed over  its  surface,  and  no  further  increase  in  rapidity 
produces  any  effect  upon  its  appearance.     Rotate  the  disk 
used  in  Ex.  145  a,  and  increase  the  rapidity  till  the  inner- 
most portion  gives  a  uniform  gray.     When  this  appears,  the 
rate  of  recurrence  in  the  outermost  ring  is  32  times  more 
rapid  than  in  the  innermost,  and  yet  no  difference  in  shade 
is  to  be  seen.     To  show  that  the  gray  is  actually  of  the 
same  brightness  that  would  come  from  an  even  distribution 
of  the  light  reflected  from  the  whole  surface  of  the  ring, 
prepare  a  disk  with  many  equal  black  and  white  sectors  — 
32  or  more  of  each.     Place  the  disk  on  the  color-mixer,  and 
look  at  it  when  at  rest  through  a  double  convex  lens  of 
short  focus  (e.g.,  1  in.),  held  at  such  a  distance  from  the  eye 
and  disk  that  no  distinct  image  is  formed,  but  the  field  of 
the  lens  appears  an  even  blur  of  gray.     Now  put  the  disk 
in  rapid   rotation  and   notice  that   the  gray  remains  un- 
changed. 

The  result  of  these  experiments  would  be  the  same  were 
other  colors  substituted  for  black  and  white. 

Helmholtz,  A,  482-485,  Fr.  446-450  (338-341);  Aubert,  A,  515-516; 
Talbot;  Plateau. 

147.  Brticke's  Experiment.     When  the  rate  of  rotation 
is  insufficient  to  produce  an  even  blending,  the  brightness 


SENSATIONS   OF  LIGHT  AND  COLOR.  147 

of  the  disk  is  influenced  by  the  rate.  Set  the  disk  used  in 
Ex.  145  a  in  rapid  enough  rotation  to  blend  the  innermost 
ring,  and  then  let  it  gradually  come  to  rest.  As  it  turns 
more  and  more  slowly,  there  will  be  observed  in  one  ring 
after  another,  beginning  with  the  innermost,  just  as  it  loses 
its  uniform  character,  a  notable  brightening.  The  white 
sectors  now  have  opportunity  to  produce  their  full  effect 
upon  the  retina  before  they  are  succeeded  and  their  impres- 
sion cut  off  by  the  black  sectors. 

Helmholtz,  A,  Fr.  455-456:  Exner;  Aubert,  A9  510. 

COLOR  MIXING. 

148.  Mixed  Colors.  Experiments  upon  this  subject  can- 
not be  regarded  as  entirely  satisfactory  except  when  made 
with  pure  (homogeneous)  spectral  colors.  The  colored 
papers  with  which  the  following  experiments  are  made 
show  anything  but  homogeneous  colors,  as  can  easily  be  seen 
by  looking  at  scraps  of  them  on  a  dark  background  through 
a  prism.  They  produce  the  same  mixture  effects,  however, 
that  spectral  colors  of  the  same  tone,  intensity,  and  satura- 
tion would  produce  ;  and  the  great  facility  of  their  manipu- 
lation on  the  color-mixer  recommends  them  for  preliminary 
experiments  and  for  illustrative  purposes. 

Three  colors  properly  selected  serve  to  produce  by  their 
mixtures  all  the  intermediate  colors  (though  in  most  cases 
in  less  saturation)  with  purple  and  white  (i.  e.,  gray)  in  addi- 
tion. The  colors  generally  selected  are  red,  green,  and  blue 
or  violet.  Green  cannot  be  mixed  from  colors  that  them- 
selves do  not  resemble  it ;  i.e.,  it  can  be  mixed  from  yellow- 
green  and  blue-green,  but  not  from  yellow  and  blue,  and  not 
in  anything  like  full  saturation. 

The  general  facts  of  color  mixing,  together  with  the 
method  of  representing  them  in  a  two  dimensional  diagram, 
were  first  discovered  by  Newton,  and  are  sometimes  desig- 


148         LABORATORY  COURSE  IN  PSYCHOLOGY. 

nated  by  the  general  term  of  Newton's  Law.  For  the 
methods  of  constructing  such  diagrams,  see,  among  others, 
Helmholtz,  A,  334  ff.,  Aubert,  A,  524  ff.,  and  Rood,  A,  218 
ff.,  224  ff. 

a.  Mix  a  yellow  from  red  and  green  on  the  color-mixer. 
The  yellow  produced  will  be  dark,  and,  as  a  test  of  its 
hue,    should   be   matched  with   a   mixture   of  yellow  and 
black  made  with  smaller  disks  set  on  above  the  first.     In 
the  same  way  mix  a  blue  from  green  and  violet  that  shall 
match  a  mixture  of  blue  and  black  (or  blue,  black,  and 
white). 

b.  From  red  and  violet  or  blue,  mix  several  purples  be- 
tween violet  arid  red. 

c.  From  red,  green,  and  violet,  mix  a  gray  that   shall 
match  a  mixture  of  black  and  white  on  the  small  disk.     In 
such  a  case   as   this   it  is  highly  probable  that   the  gray 
appears,  because  the  combined  colors  furnish  among  them 
light  of  all  wave-lengths  in  about  the  proportions  in  which 
they  occur  in  ordinary  white  light.     With  the  homogeneous 
red,  green,  and  violet  of  the  spectrum,  the  case  would  of 
course  be  different.    To  avoid  troublesome  after-images,  the 
adjustment  of  the  disks  should  be  left  to  an  assistant,  or 
the  observer  should  wear  dark  glasses,  except  when  the 
disks  are  in  revolution  at  full  speed. 

If  the  colored  disks  used  in  these  experiments  are  not 
opaque,  several  should  be  used  at  once  instead  of  a  single 
one. 

^  For  demonstrational  purposes  mixtures  of  two  colors  in 
different  proportions  can  be  shown  on  a  single  disk  of  the 
star  form  (see  Ex.  141)  by  painting  the  star  in  one  color 
and  the  ground  of  the  disk  in  another  (or  by  pasting  colored 
papers  instead  of  painting),  but  in  either  case  some  trial 
will  be  necessary  to  determine  the  proper  shape  for  the 
rays. 


SENSATIONS   OF  LIGHT  AND  COLOR.  149 

Helmholtz,  A,  311-316,  320-322,  325-333,  375,  376-473,  485,  Fr. 
359-365,  367-369,  450  (272-277,  279-281,  341);  Aubert,  A,  521-524- 
Bering,  Jf;  Maxwell,  A  and  #;  Kood,  A,  124  ff. 

149.  Complementary  Colors.  The  combination  of* red, 
green,  and  violet  mentioned  in  the  last  experiment  is  not 
the  only  combination  that  gives  white  or  gray.  For  every 
color  there  is  another  or  complementary  color,  which,  mixed 
with  it,  gives  a  colorless  combination.  Some  of  these  pairs 
are  red  and  blue-green,  yellow  and  indigo-blue,  green  and 
purple,  blue  and  orange,  violet  and  yellow-green. 

a.  Try  several  of  these  pairs  upon  the  color-mixer,  match- 
ing the  resultant  gray  with  a  mixture  of  black  and  white 
on  the  small  disk.  It  will  probably  be  found  in  some  cases 
that  no  possible  proportions  of  the  colored  papers  at  hand 
will  give  a  pure  gray.  In  that  case  a  little  of  the  color 
complementary  to  that  remaining  in  the  gray  must  be 
added.  Suppose  the  red  and  blue-green  papers,  when  com- 
bined, give  gray  with  a  tinge  of  brown  (i.e.,  dark  orange) ;  a 
certain  amount  of  blue  must  then  be  added  to  compensate. 
For  example,  with  certain  papers  180°  of  blue-green  -f-  36° 
indigo-blue  +  144°  red  make  a  gray  that  matches  90°  white 
+  270°  black.  To  see  the  true  complement  of  the  red  used, 
it  is  then  necessary  to  prepare  a  disk  carrying  green  and 
indigo  in  the  proportions  of  180  and  36;  i.e.,  300°  blue- 
green,  60°  indigo.  In  the  same  way  the  complement  of  the" 
blue-green  used  is  a  bluer  red  than  that  of  the  red  paper, 
and  may  be  seen  by  itself  by  mixing  288°  red  with  72° 
indigo.  It  is  very  important  here,  and  in  all  cases  where 
a  resultant  white  or  gray  is  to  be  observed,  to  have  some 
undoubted  white  or  gray  in  the  field  to  prevent  mistake  in 
very  faint  tinges  of  color. 

The  criticism  made  upon  Ex.  148  c  applies  here  with 
equal  force.  To  be  conclusive,  the  experiment  must  be 
made  with  far  simpler  colors  than  those  of  colored  papers. 


150         LABORATORY   COURSE  IN  PSYCHOLOGY. 

b.  Negative  after-images,  when  projected  on  a  white  sur- 
face, are  seen  in  colors  approximately  complementary  to 
those  that  give  rise  to  the  after-images.  Compare  comple- 
mentary colors  found  in  this  way  with  those  found  on  the 
color-mixer. 

Helmholtz,  A,  316-319,  Fr.  365-367  •  (277-278) ;  Aubert,  A,  521- 
524;  Konig  und  Dieterici,  A,  284  ff.  ;  Kood,  A,  161  ff. 

150.  Other  Methods  of  Mixing  Colored  Lights,  a.  Lam- 
bert's Method.  The  Reflection  Color-Mixer.  This  is  the 

simplest  of  all  the  methods. 
The  colors  to  be  mixed  are 
placed  on  a  suitable  back- 
ground (e.  g.,  a  smooth  sur- 
face of  black  velvet),  on  op- 
posite sides  of  a  vertical 
glass  plate.  The  eye  is 
brought  into  such  a  position 
that  the  reflected  image  of  the  color  on  one  side  appears  to 
overlie  that  seen  by  transmission  on  the  other  side.  The 
glass  must  of  course  be  of  good  quality  and  clean.  The 
relative  intensity  of  the  colors  can  be  varied  by  varying 
their  distance  from  the  glass.  Bringing  the  colors  near  the 
glass,  or  raising  the  eye,  strengthens  the  reflected  and  weak- 
ens the  transmitted  light.  Strips  of  colored  paper  placed 
with  their  ends  next  the  glass,  provided  the  illumination  is 
equal,  will  show  an  even  blending  of  the  colors  through  a 
considerable  range  of  intensities,  one  color  predominating 
at  one  end  of  the  combined  image,  the  other  at  the  other 
end. 

By  substituting  a  bit  of  glass  on  a  black  background  for 
one  of  the  colors,  and  then  placing  the  instrument  so  that  a 
portion  of  clear  sky  may  be  reflected  in  the  glass,  it  is  possi- 
ble to  mix  sky-blue  with  its  complement,  or  with  any  other 
color. 


SENSATIONS   OF  LIGHT  AND  COLOR. 


151 


To  mix  two  colors  in  equal  proportions,  arrange  them 
with  black  and  white,  as  in  the  diagram  below.  Adjust 
the  glass  (or  the  position  of  the  eye)  till  the  grays  made 
by  the  black  and  white  at  the  ends  exactly  match ;  the 
colors  will  then  be  mixed  in  equal  proportions. 


b.  Mixture  by  Double  Kefraction.     Colored  areas  placed 
side  by  side  appear  mixed  when  regarded  through  a  double 
refracting  prism.     The  prism  doubles  both  fields,  and  causes 
a  partial  overlapping.     In  the  overlapped  portion  the  colors 
are  mixed,  each  color  being  present  in  the  mixture  at  ap- 
proximately half  its  original  brightness.     The  prism  should 
be  achromatic. 

c.  Mixture  of  Spectral  Colors.    Fine  mixtures  may  be  ob- 
tained with  a  prism  and  Figs.  1,  2,  and  3  of  Plate  I. ;  or,  still 
better,  from  figures  shaped  like  these,  but  in  white  upon  a 
black  ground.    Since  a  prism  refracts  different  kinds  of  light 
in  different  degrees,  it  produces  a  multitude  of  partially  over- 
lapping images  of  a  bright  object,  which  appear  to  the  eye 
as  colored  fringes.     (Observe  through  a  prism  held  horizon- 
tally, an  inch  square  of  white  paper  on  a  black  background.) 
These  overlapping  images  may  be  illustrated  by  the  follow- 
ing diagram,  in  which  the  horizontal  lines  stand  for  the 


152         LABORATORY   COURSE  IN  PSYCHOLOGY. 

images,  and  the  capital  letters  for  the  colors  of  the  light 
producing  them. 

a          b 

rr  TT" 


d          c 

In  the  area  a  b  c  d  all  the  images  overlap  and  the  white 
of  the  paper  is  still  seen.  Toward  the  left  from  a,  however, 
the  different  kinds  of  light  gradually  fail,  beginning  with 
the  red.  The  successive  colors  from  greenish  blue  to  violet 
result  from  the  mixture  of  what  remains.  At  the  other 
end  a  similar  falling  away  of  the  colors  gives  the  succession 
from  greenish  yellow  to  red.  In  Fig.  1,  the  spectra  seen  on 
the  upper  and  lower  edges  of  the  inch  square  of  white 
paper  are  brought  side  by  side ;  on  one  side  red,  orange, 
and  yellow,  and  on  the  other  greenish  blue,  blue,  and  violet. 
The  colors  that  stand  side  by  side  are  complementary  pairs, 
both  in  tone,  intensity,  and  saturation;  for  the  greenish 
blue  is  the  white  of  the  paper  less  the  red,  and  the  blue  the 
same  less  the  red,  orange,  and  yellow,  and  so  with  the  rest ; 
and  if  the  two  spectra  be  exactly  superposed,  as  can  be 
done  with  an  adaptation  of  the  method,  of  b  above,  they 
will  make  precisely  the  white  from  which  they  originated. 

If  a  very  narrow  strip  of  white  upon  a  black  ground  is 
looked  at  through  the  prism,  the  images  overlap  less  and 
another  color  appears  ;  namely,  green,  as  may  be  seen  in  Fig. 
2  on  the  narrow  white  band  between  the  black  bars.  WJien, 
on  the  other  hand,  a  narrow  black  band  on  a  white  ground 
is  taken,  the  spectrum  of  the  white  surface  above  and  of 
that  below  partially  overlap,  and  give  another  set  of  mix- 
tures. If  the  diagram  is  held  near  the  prism  at  first,  and 


SENSATIONS   OF  LIGHT  AND   COLOR.  153 

then  gradually  withdrawn  from  it,  the  advance  and  mixing 
of  the  spectra  can  easily  be  followed.  Besides  the  greenish 
yellow  at  one  end  and  the  greenish  blue  at  the  other,  there 
are  a  rich, purple,  complementary  to  the  green  beside  it,  and 
a  white  between  the  purple  and  the  greenish  yellow.  The 
last  is  a  white  produced  by  the  mixture  of  the  blue  of  one 
spectrum  with  the  complementary  orange-yellow  of  the 
other. 

Fig.  3  shows  a  number  of  color  mixtures  with  different 
proportions  of  the  constituents.  In  the  spectra  from  the 
white  triangle  appear  mixtures  of  each  color  in  the  spectrum 
seen  on  the  white  band  in  Fig.  2,  with  every  other  color 
found  there.  Upon  the  black  triangle  the  spectra  from  the 
white  edges  above  and  below  show  mixtures  similar  to 
those  on  the  black  band  in  Fig.  2.  The  diagram  should  be 
placed  at  such  a  distance  that  a  little  of  the  white  and  black 
triangles  can  still  be  seen. 

Helmholtz,  A,  350-357,  485,  491-493,  Fr.  402-407,  450,  458-461 
(303-306,  341,  347-349);  Aubert,  .4,521-524  ;  Maxwell,  A\  Rood,  A, 
108  ff.,  124  ff.;  Hering,  O;  von  Bezold,  B,  77  ff.  On  a  and  c,  Ben- 
son. On  refined  methods  of  mixing  spectral  colors,  see  especially 
the  first  reference  to  Helmholtz. 

CONTRAST. 

The  effect  of  one  color  on  another,  when  not  mixed  with 
it,  but  presented  to  the  eye  successively,  or  simultaneously 
in  adjacent  fields,  is  known  as  contrast.  Two  kinds  are 
distinguished,  Successive  contrast  and  Simultaneous  contrast. 
The  color  that  is  changed  or  caused  to  appear  upon  a  color- 
less surface,  is  known  as  the  induced  color ;  the  color  that 
causes  the  change  is  called  the  inducing  color.  Successive  con- 
trast is  largely  a  matter  of  negative  after-images,  and  their 
projection  upon  different  backgrounds,  and  is  universally 
regarded  as  a  matter  of  physiology.  Simultaneous  contrast, 
on  the  contrary,  has  been  regarded  by  Helmholtz  and  his 


154         LABORATORY  COURSE  IN  PSYCHOLOGY. 

supporters  as  a  matter  of  psychology,  as  a  sort  of  mis- 
judgment.  The  studies  of  the  last  few  years,  however, 
chiefly  those  of  Hering,  have  demonstrated  that  simultane- 
ous contrast  also  in  most,  and  probably  in  all  cases,  is 
physiological,  a  phenomenon  of  the  retina  (and  its  central 
connections),  not  of  mistaken  inference. 

151.  Successive  Contrast,  a.  Prepare  a  set  of  colored 
fields  of  the  principal  colors,  including  white,  black,  and 
gray,  say  3x5  inches  in  size,  and  some  small  bits  of  the 
same  colors,  say  1  cm.  square.  Lay  a  small  square  on  the 
black  field,  get  a  strong  negative  after-image,  and  project 
it  first  on  the  white  and  then  on  the  other  fields.  Notice 
that  the  color  of  the  after-image  spot  is  that  of  the  field  on 
which  it  is  projected,  minus  the  color  that  produced  the 
spot ;  e.  g.,  the  after-image  of  red  projected  on  violet  looks 
blue,  and  on  orange  looks  yellow.  Or,  to  say  the  same 
thing  in  other  words,  the  color  of  the  spot  is  a  mixture  of 
the  color  of  the  after-image  with  the  color  of  the  ground 
upon  which  it  is  projected.  Thus  a  blue-green  after-image 
when  projected  on  violet,  gives  blue ;  when  projected  on 
orange,  gives  yellow.  Notice  that  when  the  image  is  pro- 
jected on  a  field  of  the  inducing  color  it  causes  the  spot  on 
which  it  rests  to  look  dull  and  faded ;  but  when  it  is  pro- 
jected upon  a  field  of  complementary  color,  it  makes  the 
spot  richer  and  more  saturated.  Indeed,  it  is  only  by  first 
fatiguing  the  eye  for  one  color  and  then  looking  at  its  com- 
plement that  the  most  saturated  color  sensations  can  be 
produced.  In  general,  colors  that  are  complementary,  or 
nearly  so,  are  helped  in  appearance  by  contrast ;  those  that 
resemble  each  other  more  nearly  are  injured. 

b.  These  effects,  in  even  greater  brilliancy,  can  be  seen 
by  laying  the  small  square  of  color  directly  on  the  larger 
colored  surface,  staring  at  it  a  few  seconds,  and  then  sud- 
denly puffing  it  away  with  the  breath.  See  also  Ex.  134. 


SENSATIONS    OF  LIGHT  AND   COLOR.  155 

c.  This  contrast  effect  may  be  so  strong  as  actually  to 
overcome  a  moderately  strong  objective  color.  Place  a 
small  piece  of  opaque  orange  paper  in  the  middle  of  a  pane 
of  red  glass  and  look  through  the  glass  at  a  clear  sky  or 
bright  cloud.  The  strength  of  the  induced  blue-green  will 
be  sufficient  to  make  the  orange  seem  blue.  See  also  Ex. 
124  d. 

Helmholtz,  A,  537-542,  Fr.  510-515  (388-392);  Hess,  C ;  Rood,  A, 
235  ff. 

152.  Mixed  Contrasts.  When  special  precautions  are 
not  taken  to  exclude  successive  contrast,  both  successive 
and  simultaneous  co-operate  in  the  general  effect.  Some  of 
the  results  are  striking  and  beautiful. 

a.  Colored  Shadows.     Arrange  two  lights  so  that  they 
shall  cast  a  double  shadow  of  a  pencil  or  small  rod  upon  a 
white  surface.     The  daylight  will  answer  for  one  light  if  it 
is  not  too  strong,  but  it  must  not  be  forgotten  that  unless 
the  light  comes  from  an  overcast  sky  it  will  be  blue.     In- 
troduce different  colored  glasses  one  after  another  before 
one  of  the  lights,  and  notice  the  beautiful  complementary 
color  that  immediately  appears  in  the  shadow  belonging  to 
that  light.     The  brightness  of  the  two  lights  should  be  so 
regulated  that  the  shadows  shall   be  about  equally  dark 
when  the   colored   glass  is  introduced  before  one  of  the 
lights.     See  also  Ex.  155. 

Use  a  blue  glass,  and  adjust  the  relative  intensities  of  the 
lights  so  that  the  yellow  shadow  appears  at  its  brightest, 
and  notice  that  it  seems  as  bright  as  the  surrounding  blue, 
or  even  brighter.  As  a  matter  of  fact,  however,  it  receives 
less  light  than  the  surrounding  portions  ;  for  in  order  to  be 
a  shadow,  it  must  be  a  portion  of  the  field  from  which  the 
light  is  partly  cut  off. 

b.  Mirror  Contrasts.     Eagona  Scina's  Experiment.    Place 
upon  the  horizontal  and  vertical  surfaces  of  the  instrument 


156         LABORATORY  COURSE  IN  PSYCHOLOGY. 

white  cards  carrying  black  diagrams.1  The  diagrams  being 
in  place,  hold  between  the  two  at  an  angle  of  45°  a  pane  of 
colored  glass,  say  green,  and  observe  that  the  black  of  the 
horizontal  diagram  seems  tinged  with  the  complementary 
color,  that  is,  purple.  This  contrast  color  may  often  be  im- 
proved by  slightly  altering  the  inclination  of  the  glass,  or 
by  changing  the  relative  illumination  of  the  diagrams  by 
interposing  a  colorless  screen  between  one  or  the  other  of 
them  and  the  source  of  light,  or  by  shifting  the  whole  in- 
strument. This  experiment  will  be  readily  understood  after 

a  consideration  of  the  accompany- 
ing cut.  The  glass  plate  is  repre- 
sented by  C  D,  the  black  portion 
of  the  vertical  diagram  by  the 
projection  opposite  A,  that  of  the 
horizontal  diagram  by  the  projec- 
tion at  B.  The  light  reaching  the 
eye  from  the  white  portion  of  the 
horizontal  diagram  is  colored  green 
by  the  glass ;  that  from  the  white 
portion  of  the  vertical  diagram  is  reflected  from  the  upper 
surface  of  the  plate,  and  is  therefore  uncolored.2  The  mix- 
ture of  the  two  gives  a  light  green  field.  For  simplicity, 
we  may  assume  that  no  light  comes  from  the  black  portions 
of  the  diagram.  Then  in  the  portion  of  the  light  green 

1  Any  black  spot  will  answer.  For  this  experiment  diagrams  made  up  of  sets 
of  heavy  concentric  black  rings,  lines  a  quarter  of  an  inch  wide,  separated  by 
white  rings  of  triple  width,  give  an  excellent  effect.  The  diameters  should  be  so 
chosen  that  a  black  ring  on  the  horizontal  diagram  shall  correspond  to  a  white 
one  on  the  vertical  and  vice  versa,  and  shall  appear  to  lie  in  the  midst  of  the 
white  when  the  diagrams  are  combined  in  the  way  described  above.  A  pair  of 
diagrams  made  up  of  parallel  black  bars,  a  quarter  of  an  inch  wide,  separated 
by  quarter  inch  spaces,  and  so  placed  in  the  instrument  that  they  give  a  checker- 
board pattern  when  combined,  are  useful  for  keeping  in  the  field  a  true  black 
with  which  the  changed  colors  can  be  compared. 

1  As  a  matter  of  fact,  a  small  portion  is  also  reflected  from  the  lower  surface 
of  the  glass,  and  contributes  a  minute  amount  of  green. 


SENSATIONS  OF  LIGHT  AND    COLOR.  157 

field  corresponding  to  the  black  of  tlie  vertical  diagram,  the 
white  component  will  be  wanting  and  the  green  will  appear 
undiluted ;  in  the  portion  corresponding  to  the  black  of  the 
horizontal  diagram,  the  green  component  will  be  wanting 
and  the  faint  white  (i.  e.,  gray)  should  appear  by  itself. 
It  does  not,  however,  because  of  the  contrast  color  induced 
upon  it.  As  a  matter  of  fact,  the  black  portions  are  not 
absolutely  black ;  the  small  amount  of  light  that  comes 
from  them  tends  on  one  hand  to  make  the  green  image  (im- 
age of  the  black  of  the  vertical  diagram)  a  little  whiter,  and 
on  the  other  hand  to  counteract  the  contrast  in  the  purple 
image  by  adding  to  it  a  little  green.  Try  the  experiment 
with  other  glasses  than  green. 

Another  form  of  the  mirror  contrast  experiment  is  as 
follows.  Place  a  mirror  where  the  sky  or  a  white  surface 
of  some  kind  will  be  seen  reflected  in  it.  Lay  upon  its  sur- 
face a  plate  of  colored  glass  (green  for  example),  and  hold 
a  little  way  above  it  a  narrow  strip  of  black  cardboard  or  a 
pencil.  Two  images  will  be  seen :  one  a  vivid  green,  the 
other  a  complementary  purple.  The  green  image  belongs 
to  the  surface  reflection  of  the  colored  glass,  as  may  be 
proved  by  observing  that  when  the  strip  of  cardboard 
touches  the  surface,  the  green  image  touches  it  also.  The 
purple  image  belongs  to  the  reflection  from  the  back  of 
the  mirror.  It  is  easy,  by  substituting  a  gray  strip  for  the 
black,  to  show  that  contrast  can  suppress  a  weaker  objective 
color  actually  present.1 

c.  Meyer's  Experiment.  Lay  on  a  large  colored  field  a 
small  piece  of  gray  or  even  black  paper  (e.g.,  1  cm.  wide  by 
2  cm.  long),  and  cover  the  whole  with  a  piece  of  semi- 
transparent  white  paper  of  the  same  size  as  the  colored 
field.  The  contrast  color  will  appear  on  the  gray  paper. 

1  For  fuller  explanation  with  diagram,  see  American  Journal  of  Psychology  t 
V.,  1892-93,  407,  and  von  Bezold,  154  f. 


158        LABORATORY  COURSE  IN  PSYCHOLOGY. 

If  thin  tissue  paper  is  used,  more  than  one  thickness  may 
be  needed  for  the  best  result.  Paper  mats,  woven  one  way 
of  gray  paper  and  the  other  of  colored,  show  this  contrast 
beautifully.  They  may  easily  be  made  from  kindergarten 
materials. 

d.    Mixed  Contrasts  with  the  Color-mixer.    Disks  made  on 
the  pattern  of  the  cut  at  the  left  show  beautiful  contrasting 


grays.  The  disk  used  in  Ex.  145  c  shows  a  longer  series, 
but  requires  a  more  rapid  rate  of  rotation.  The  same  can 
be  shown  also  by  laying  a  number  of  small  sheets  of  tissue 
paper  over  one  another  in  such  a  way  that  they  partially 
overlap,  making  a  portion  where  there  is  but  a  single  thick- 
ness, and  next  it  a  portion  where  there  are  two  thicknesses, 
and  next  that  again  one  of  three  thicknesses,  and  so  on. 
When  the  whole  is  held  up  to  the  light,  the  contrasts  of 
adjacent  portions  are  very  easily  seen. 

Contrast  colors  can  be  shown  finely  with  disks  like  that 
in  the  cut  at  the  right,  in  which  the  shaded  portions  repre- 
sent color,  the  black  portions,  black,  and  the  white,  white. 
A  little  care  is  necessary  in  fixing  the  proportions  of  the 
color  to  white  and  black  in  the  disks,  but  in  general  the 


SENSATIONS  OF  LIGHT  AND   COLOR.  159 

brightness  of  the  gray  should  be  about  that  of  the  color. 
When  the  contrast  color  has  been  satisfactorily  obtained, 
bring  near  it  a  piece  of  white  cardboard  (e.g.,  3  x  5  in.),  so 
held  with  reference  to  the  source  of  light  that  it  appears 
about  as  bright  as  the  contrast  ring.  Hold  the  card  so  that 
its  shadow  does  not  fall  on  the  disk,  or  at  least  is  out  of 
sight.  Notice  the  retreat  of  the  contrast  color  from  its 
edges.  On  such  experiments  as  this  much  stress  is  laid  by 
Helrnholtz  and  the  supporters  of  the  psychological  explana- 
tion of  contrast. 

Contrasts  with  two  colors  at  once  can  be  shown  by  mak- 
ing the  inner  portion  of  the  colored  sectors  of  one  color,  the 
outer  portion  of  another.  A  temporary  disk  for  showing 
contrast  effects  may  be  arranged  by  putting  on  the  spindle 
of  the  color-mixer  first  a  large  colored  disk  (e.g.,  20  cm. 
.  in  diameter),  then  smaller  combined  disks  of  black  and 
white  (e.g.,  12  cm.  in  diameter),  and  finally  a  still  smaller 
colored  disk  (e.g.,  10  cm.  in  diameter). 

Helmholtz,  A,  542  ff.,  Fr.  515-546  (392-417);  Hering,  j£;  Aubert, 
496  if.,  546  ff.;  von  Bezold,  144-171;  Rood,  241-272;  Mayer. 

For  particular  experiments,  see  the  following:  on  a  (second  part), 
von  Bezold,  B,  153-154;  on  b  (second  part),  Dove;  on  c,  Meyer. 

For  quantitative  measurements  of  contrast  in  grays,  see  Ebbing- 
haus,  B  ;  Lehmann;  and  Kirschmann,  D. 

153.    Some  of  the  Conditions  that  Influence  Contrast. 

a.  Contrasts  are  stronger  when  the  colors  are  near  to- 
gether. Lay  a  bit  of  white  paper  on  a  black  surface,  e.g., 
a  piece  of  black  velvet,  and  notice  that  the  paper  is  whiter 
and  the  velvet  blacker  near  the  margin  of  the  paper  than 
elsewhere,  notwithstanding  that  the  eye  moves  about  freely. 
This  has  received  the  name  of  "  Marginal  contrast "  (Rand- 
contrast). 

On  a  piece  of  gray  paper,  the  size  of  a  letter-sheet,  lay  two 
strips  of  colored  paper  close  side  by  side  (e.g.,  pieces  of 


160        LABORATORY  COURSE  IN  PSYCHOLOGY. 

red  and  yellow  or  of  green  and  blue,  1  cm.  wide  by  4  cm. 
long).  Below  them  to  the  right  and  left,  as  far  apart  as  the 
paper  will  permit,  lay  two  other  strips  of  the  same  size  and 
color,  red  on  the  red  side  of  the  former  pair,  yellow  on  the 
yellow  side.  Notice  the  effect  of  the  difference  in  distance 
on  the  contrasting  pairs.  Contrast  of  this  sort  is  at  a  maxi- 
mum when  one  color  entirely  surrounds  the  other. 

b.  Effect  of  size.     When  the  area  of  the  inducing  color  is 
large  and  that  of  the  induced  color  is  small,  the  contrast 
is  shown  chiefly  on  the  latter ;   when  the  two  areas  are  of 
about  equal  size,  as  in  a  above,  the  effect  is  mutual.     Try 
with  large  and  small  bits  of  paper  upon  a  colored  field. 

c.  Borders    and  lines   of  demarcation  that  separate  the 
contrasting  areas  tend  to  lessen  the  effect  by  excluding  mar- 
ginal contrast ;   and   (since   the  eye  tends  to  move  along 
rather  than  across  strongly  marked  lines),  by  hindering  such 
motions  of  the  eye  as  would  bring  about  successive  contrast. 
Eepeat  Ex.  152  c,  using  two  slips  of  gray  paper  5  mm.  wide 
by  2  cm.  long,  and  substituting  a  piece  of  moderately  trans- 
parent letter-paper  for  the  tissue  paper.    When  the  contrast 
color  has  been  observed,  trace  the  outline  of  one  of  the 
slips  with  a  fine  ink  line  upon  the  paper  that  covers  it,  and 
notice  that  the  color  nearly  or  quite  vanishes.     A  disk  like 
that  in  the  cut  accompanying  Ex.  152  d,  when  provided  with 
a  second  contrast  ring,  marked  off  on  both  its  edges  with  a 
firm  black  line,  shows  a  weakening  of  the  induced  color  in 
the  bordered  ring. 

This  experiment  and  others  like  it  play  an  important  part 
in  the  psychological,  as  opposed  to  the  physiological,  expla- 
nation of  simultaneous  contrast ;  see  Helmholtz,  A,  543  ff., 
559  f.,  Fr.  533  f.,  539,  542,  (406  f.,  411,  414).  Such  a  black 
border  will,  however,  also  make  a  weak  objective  color 
invisible. 

d.  Saturation.     Contrast  effects  are  generally  most  strik- 


SENSATIONS   OF  LIGHT  AND  COLOR.  161 

ing  with  little  saturated  colors.  Compare  the  effect  of 
increasing,  decreasing,  and  extinguishing  the  second  non- 
colored  light  in  the  colored  shadow  experiments.  It  is 
necessary,  however,  to  see  to  it  that  reflected  light  from  the 
walls  and  surrounding  objects  does  not  complicate  the  ex- 
periment. 

Compare  the  intensity  of  the  contrasts  in  Meyer's  experi- 
ment (Ex.  152  c)  before  and  after  the  application  of  the 
tissue  paper.  Notice  also  the  part  played  by  the  white 
light  mixed  with  the  colored  light  in  the  mirror  contrast 
experiments  above.  Try  the  effect  of  introducing  white  or 
black  or  both  into  the  largest  and  smallest  disks  in  the 
arrangement  mentioned  at  the  end  of  Ex.  152.  Powerful 
contrasts  with  the  most  saturated  colors  can  be  observed, 
however,  when  the  proper  conditions  are  fulfilled. 

e.  Colors,  induced  upon  gray  fields  are  stronger  when  the. 
gray  has  about  the  same  brightness  as  the  inducing  color. 
Eepeat  Meyer's  experiment,  using  white  paper  instead  of 
the  gray  or  black.  With  the  three  disk  arrangement  try 
the  effect  of  making  the  intermediate  disk  all  white  and  all 
black.  Eood  finds  that  grays  slightly  darker  than  the 
inducing  color  are  advantageous  when  the  inducing  color 
is  red,  orange,  or  yellow,  and  slightly  lighter  when  the 
inducing  color  is  green,  blue,  violet,  or  purple. 

On  conditions  in  general,  see  Helmholtz,  A,  540-541,  Fr.  513-514, 
(390-391),  Kirschmann,  D.  In  Hering,  E,  will  also  be  found  much  on 
the  effect  of  various  conditions.  On  ft,  Exner,  B.  On  c,  Helmholtz, 
A,  546-547,  Fr.  539-542  (411-414).  On  d,  Helmholtz,  A,  Fr.  523-524 
(399-400).  On  e,  Hood,  A,  261. 

154.  The  Halo  or  Lichthof  of  Hering.  Contrast  is  often 
to  be  seen  in  negative  after-images.  That  observed  in  after- 
images of  white  objects  on  a  dark  ground  has  been  adduced 
by  Hering  as  an  argument  against  the  psychological  expla- 
nation of  contrast.  Some  of  the  simpler  experiments  are 


162        LABORATORY  COURSE  IN  PSYCHOLOGY. 

as  follows ;  for  his  development  of  them  consult  Hering,  A. 

a.  Lay  a  half  inch  square  of  white  paper  on  a  large  sheet 
of  black  cardboard  (or  better  of  black  velvet),  and  put  a 
small  dot  at  its  centre.     Stare  with  unmoved  eyes  at  the 
dot  for  from  15  to  30  seconds  or  more,  then  close  and  cover 
the  eyes.     There  will  then  be  seen,  neglecting  incidental 
color  effects,  the  dark  after-image  of  the  paper  surrounded 
by  a  halo  of  light,  brightest  next  the  paper  and  gradually 
falling  off  in  brilliancy  toward  the  periphery.     This  is  ex- 
plained on  the  psychological  theory  as  due  to  contrast  with 
the  deep  black  of  the  after-image  of  the  square.     When, 
however,  the  converse  of  the  experiment  is  properly  made 
(a  black  square  on  a  white  ground),  the  dark  halo  which 
would  be  expected  by  contrast  is  not  found,  though  the 
after-image  of  the  black  square  is  very  bright. 

b.  Lay  two  white  squares  side  by  side  two  or  three  milli- 
meters apart  on  the   dark   ground   and   between  them  a 
minute  clipping  of  paper  for  a  fixation  point.     Secure  the 
after-images  as   before.    .  The   halos   qf   the   two   squares 
coincide  in  the  narrow  space  between  and  give  a   much 
brighter  band  in  the  after-image.     Under  favorable  circum- 
stances this  bright  band  may  remain  visible  while  the  after- 
images of  the  squares  themselves  are  temporarily  invisible. 
In  both  these  experiments  it  is  better  to  use  both  eyes  than  a 
single  one.     The  explanation  of  the  halo  as  a  matter  of  false 
judgment,  especially  in  the  last  mentioned  case,  is  not  easy. 

Hering,  A. 

155.  Simultaneous  Contrast  with  Colored  Shadows.  The 
effects  of  simultaneous  contrast  are  almost  always  lost  in  the 
more  powerful  ones  of  successive  contrast.  The  first  requi- 
site, therefore,  of  an  experiment  on  the  first,  is  the  exclusion 
of  the  second.  This  is  not  difficult  for  colored  shadows. 

a.  Place  a  good-sized  piece  of  white  paper  on  a  table  in 
such  a  position  that  it  may  be  illuminated  at  the  same 


SENSATIONS   OF  LIGHT  AND   COLOR.  163 

time  from  a  window  (if  the  day  is  overcast)  and  from  a  gas- 
jet.  Set  upon  it  a  small  block  or  other  object  (about  5  cm. 
by  10  cm.  in  size)  ;  something  black  in  color  is  best.  Light 
the  gas  and  observe  the  two  shadows,  one  cast  by  the  light 
from  the  window,  the  other  by  the  gas.  The  first  will 
appear  yellowish,  the  second  clearly  blue.1  Adjust  the  dis- 
tance and  position  of  the  block  with  reference  to  the  light 
so  that  the  shadows  shall  appear  about  equally  dark,  and 
the  blue  shadow  shall  be  as  sharply  bounded  as  possible, 
and  for  that  purpose  it  is  well  to  have  the  shadow  cast  by 
the  edge  rather  than  the  flat  side  of  the  flame.  The  color 
of  the  yellowish  shadow  is  objective  and  due  to  the  yellow 
of  the  gas-flame,  that  of  the  blue  is  due  to  the  contrast,  but 
largely,  as  yet,  to  successive  contrast.  Put  a  dot  in  the 
centre  of  the  blue  shadow,  to  serve  as  a  fixation-point,  and 
another  on  the  edge.  Fasten  a  paper  tube  (preferably 
blackened  inside)  so  that  it  can  easily  be  shifted  from  one 
dot  to  the  other.  Cut  off  the  gas-light  by  holding  a  card 
between  it  and  the  block  ;  adjust  the  tube  so  that  the  dot  in 
the  middle  of  the  shadow  may  be  fixated  without  any  of 
the  field  outside  of  the  shadow  being  seen.  Wait  until  all 
of  the  blue  has  disappeared  from  the  shadow,  and  then, 
still  looking  through  the  tube,  remove  the  card.  The  field 
remains  entirely  unchanged  and  appears,  as  before,  a  color- 
less gray.  The  former  blue  color  is  thus  shown  to  be  sub- 
jective and  due  to  contrast  with  the  yellow  lighted  area  in 
which  it  lies. 

1  This  setting  of  the  experiment  succeeds  best  when  the  daylight  is  weak,  as, 
for  example,  just  before  the  lights  are  usually  lighted  in  the  evening.  If  the  ex- 
periment is  to  be  made  in  broad  day,  the  light  must  be  reduced  by  curtains  or 
otherwise;  if  at  night,  there  must  be  two  lights,  one  corresponding  to  the  win- 
dow  and  one  to  the  gas,  and  the  latter  must  shine  through  a  pane  of  colored  glass. 
If  yellow  glass  is  used,  the  colors  will  be  the  same  as  those  in  this  experiment,  the 
free  flame  taking  the  place  of  the  daylight.  If  the  sky  is  clear,  its  light  is  itself 
blue,  and  would  complicate  the  experiment  somewhat.  Its  light  may,  however, 
be  passed  through  colored  glass  or  gelatine,  but  then  the  orange  color  of  the 
gas-light  must  be  regarded. 


164        LABOEATOllY  COURSE  IN  PSYCHOLOGY. 

b.  Cut  off  the  gas-light  again  and  adjust  the  tube  so  that 
the  dot  in  the  edge  of  the  shadow  may  be  fixated.  Taking 
great  care  not  to  move  the  eye,  withdraw  the  card.  The 
part  of  the  field  of  the  tube  filled  by  the  shadow  will  ap- 
pear bluish,  that  of  the  remainder  reddish  yellow.  After 
a  little  time  of  steady  fixation,  cut  off  the  gas-light  once 
more  and  observe  the  instant  reversal  of  the  colors.  The 
shadow  now  appears  in  reddish  yellow,  the  rest  of  the  field 
blue.  The  color  of  the  shadow,  both  before  and  after  the 
final  interposition  of  the  card,  is  due  to  simultaneous  con- 
trast, in  the  first  case  with  the  reddish  yellow  light,  and  in 
the  second  with  its  after-image. 

Helmholtz  and  his  supporters  explain  all  cases  of  simul- 
taneous contrast  as  errors  of  judgment ;  in  the  case  of  the 
colored  shadow,  for  example,  we  mistake  the  yellow  of  the 
gas-lighted  field  for  white,  and  consequently  find  the  shadow 
which  is  really  gray  to  be  bluish.  In  the  case  of  this  par- 
ticular experiment,  Hering  and  Delabarre  have  shown  this 
psychological  explanation  unnecessary  and  a  physiological 
one  all  sufficient,  and  Hering  has  done  the  same  for  other 
forms  of  experiments. 

On  simultaneous  contrast  in  general,  see  Helmholtz,  J.,  542  ff., 
Fr.  515-547  (392-418) ;  Hering,  A  and  E.  On  colored  shadows  see 
Helmholtz,  A,  551-553,  Fr.  517-519  (394-396) ;  Hering,  E  ;  Delabarre. 

On  Helmholtz's  theory  see  Helmholtz,  J.,  543  ff.,  Fr.  516,  533-538 
(392,  407-411);  Hering,  E ;  Rood,  A,  252  ff.;  von  Bezold,  B,  146  ff. 

For  quantitative  measurements  of  simultaneous  contrast  under 
various  conditions,  see  Kirschmann,  D. 

156.    Simultaneous  Contrast.   Hering's  Binocular  Method. 

a.  Set  a  red  glass  in  the  right  frame  of  the  binocular 
color-mixer,  a  blue  glass  in  the  left.  Look  fixedly  through 
the  colored  glasses  at  the  cork  ball  below,  bringing  the  eyes 
close  to  the  glasses  and  the  nose  between  them.  Adjust  the 
side  screens  till  the  white  ground  below  appears  in  a  uni- 


SENSATIONS   OF  LIGHT  AND  COLOR.  165 

form  light  violet  from  the  binocular  mixture  of  the  red  and 
blue  (see  Ex.  167).  The  narrow  strip  of  black  paper  on  the 
white  is  seen  double,  the  right  hand  image  bluish,  the  left 
yellowish. 

b.  The  possibility  of  successive  contrast,  however,  is  not 
yet  excluded.  Lay  a  sheet  of  black  paper  over  the  whole 
of  the  white  field  and  its  black  strip ;  rest  the  eyes ;  and 
finally,  when  everything  is  in  readiness,  and  the  eyes  again 
fixed  on  the  ball,  swiftly  draw  away  the  black  paper,  keep- 
ing the  eyes  motionless.  The  contrast  colors  are  seen  on 
the  instant,  before  any  motions  of  the  eyes  that  might  intro- 
duce successive  contrast  have  been  made. 

Hering  argues  that  this  experiment  is  conclusive  against 
the  psychological  explanation  of  simultaneous  contrast, 
unless  a  separate  unconscious  judgment  is  to  be  made  for 
each  eye ;  for  that  which  is  seen  is  a  light  violet  field, 
and  the  contrast  color  to  that  should  be  a  greenish  yellow, 
and  both  images  of  the  strip  should  be  alike,  whereas, 
actually,  the  images  appear  in  different  colors,  neither  of 
which  is  the  color  required. 

Hering,  J. 

157.  Induction  of  a  Like  Color.  An  effect  the  reverse 
of  the  ordinary  contrast  effects  sometimes  appears,  the  in- 
ducing color  reappearing  in  the  induced  field. 

a.  Place  close  side  by  side  a  large  piece  of  black  paper 
and  an  equal  sized  piece  of  white.     Make  a  dot  as  a  fixation 
point  at  the  middle  of  their  line  of  junction,  and   stare 
fixedly  at  it  for  half  a  minute.     After  a  few  seconds  the 
white  will  appear  decidedly  darker  and  the  black  decidedly 
lighter,   the   effect   becoming   more  marked  as  fixation  is 
continued.     See  also  Ex.  122. 

b.  A  darkening  or  brightening  of  a  colored  ground  is  often 
to  be  observed  when  a  figure  in  black  or  white  is  placed 


166        LABORATORY  COURSE  IN  PSYCHOLOGY. 

upon  it.  This  is  a  method  of  obtaining  shades  and  tints 
often  used  in  polychromatic  decoration.  Observe  the  effect 
in  Fig.  4  of  Plate  I.  The  same  may  be  observed  occasion- 
ally in  plaid  fabrics,  and  is  shown  very  satisfactorily  in 
kindergarten  mats  woven  in  checker-board  pattern  of  col- 
ored and  gray  papers.  If  a  set  of  graded  grays  is  used  so 
that  the  strips  may  range  evenly  from  a  black  at  one  side 
to  a  white  at  the  other,  the  corresponding  shading  of  the 
colored  paper  is  striking. 

On  a,  Helmholtz,  A,  554  ff.,  Fr.  527  ff.  (401  ff.);  Hering,  A,  36  ff. 
On  &,  von  Bezold,  I?,  182-183  and  Plate  Y.  For  what  is  perhaps  a 
related  phenomenon,  see  Briicke,  424  ff . ;  Helmholtz,  A,  549,  Fr. 
520  (396);  Aubert,  A,  549  f. 

158.  Influence  of  Experience  in  Visual  Perception.  While 
in  the  previous  experiments  a  physiological  explanation 
seems  sufficient  for  the  facts,  psychical  action  is  not  ex- 
cluded, even  by  Hering,  from  a  considerable  share  in  sense 
perception.  In  the  following  experiments  experience  co- 
operates in  the  result. 

a.  Place  upon  the  color-mixer  a  short-pointed  star  of 
white  cardboard,  or  even  a  square ;  when  in  sufficiently 
rapid  rotation,  it  appears  as  a  white  central  circle  sur- 
rounded by  a  more  or  less  transparent  ring.  While  in  this 
condition  bring  behind  it  a  broad  strip  of  black  cardboard 
of  somewhat  greater  length  than  the  diameter  of  the  star 
from  point  to  point.  As  the  edge  of  the  card  advances,  it 
can  be  seen  not  only  behind  the  transparent  ring,  but,  appar- 
ently, also  behind  the  opaque  central  circle,  and  the  portions 
of  the  latter  in  front  of  the  black  card  seem  darkened  by 
its  presence.  The  illusion  holds,  though  with  a  lightening 
instead  of  a  darkening  effect,  when  a  white  card  is  moved 
behind  a  black  star.  The  illusion  fails  by  degrees  if  the 
card  is  kept  motionless,  but  may  be  observed  to  a  certain 
extent  when  the  star  is  at  rest,  or  even  on  a  square  of  card- 


SENSATIONS  OF  LIGHT  AND   COLOR.  167 

board  held  in  the  hand  while  another  is  moved  to  and  fro 
behind  it.  In  all  cases  the  latter  card  should  often  be 
wholly  withdrawn,  so  that  its  edge  can  be  clearly  seen. 

b.  Cover  a  piece  of  black  cardboard  smoothly  with  tissue 
paper,  and  notice  that  it  seems  at  first  blacker  (because  its 
color  is  well  known)  than  it  afterwards  proves  to  be  on  com- 
parison with  other  grays. 

c.  In  mixing  colors  by  reflection  (Ex.  150  a),  notice  the 
tendency  to  see  one  color  through  the  other,  instead  of  see- 
ing the  mixture  of  the  two.     This   tendency  may  be   so 
strong  at  first  as  to  interfere,  to  a  certain  extent,  with  the 
success  of  the  experiment.     See  also  Ex.  164. 

Helmholtz,  A,  312,  323  f.,  Fr.  360  (273);  Kirschmann,  E.  On  the 
difficulty  of  judging  small  differences  in  the  color  of  surfaces  that 
present  other  small  unlikenesses,  see  Hering,  E. 

SOME  PHENOMENA  OP  KOTATING  DISKS. 

159.  The  Munsterberg-Jastrow  Phenomenon,  a.  Set  a 
black  and  white  disk,  e.g.,  that  used  in  Ex.  145  a,  in  rapid 
enough  rotation  to  give  a  uniform  gray;  pass  rapidly  before 
it  a  thin  wooden  rod  or  thick  wire,  and  notice  the  multitude 
of  shadowy  images  of  the  rod  that  appear  on  the  disk.  The 
number  of  images  is  greatest  in  the  portion  of  the  disk 
having  the  most  frequent  interchange  of  black  and  white. 

b.  Replace  the  disk  by  one  carrying  two  or  more  colors. 
Notice  the  repetition  of  the  phenomenon,  and  that  the 
colors  of  the  images  are  the  colors  (otherwise  completely 
blended)  which  the  disk  actually  carries.  The  explanation 
of  the  phenomenon  is  not  altogether  clear,  but  the  sudden 
changes  of  the  background  against  which  the  rod  is  seen 
seem  to  have  an  effect  not  unlike  that  of  a  stroboscopic  disk 
or  of  intermittent  illumination,  and  thus  show  the  rod  at 
rest  in  its  successive  positions. 

Jastrow. 


168        LABORATORY  COURSE  IN  PSYCHOLOGY. 

160.  Eetinal  Oscillation.     Prepare  a  disk  of  black  card- 
board 25-30  cm.  in  diameter,  and  paste  upon  it  a  sector  of 
white  of  90°  extent.     Put  the  disk  in  slow  rotation  (one  turn 
a  second),  fixate  the  middle  of  the  disk,  and  notice  that  the 
retreating  edge  of  the  black  is  always  followed  by  a  narrow 
shadowy  sector  in  the  white.     Under  favorable  conditions 
more  than  one  may  be  seen.     The  retina  on  first  being  stimu- 
lated with  white,  apparently  reacts  in  the  direction  of  black 
(see  Ex.  125),  then  swings  again  toward  white,  and  so  on. 

Charpentier,  B. 

161.  Perception  of  Flicker  with  Different  Parts  of  the 
Eetina.     Place  upon  the  color-mixer  a  black  and  white  disk 
in  which  the  sectors  are  complete  from  centre  to  circumfer- 
ence ;  those  used  in  Ex.  145  will  not  answer  here.     Eotate 
the  disk  at  such  a  rate  as  to  give  a  lively  flicker,  fixate  its 
centre  and  slowly  increase  the  rate.     With  care  a  point  will 
be  found  where  the  sectors  are  blended  for  the  central  parts 
of  the  retina,  but  still  flicker  for  the  periphery.     Try  also 
looking  at  one  edge  of  the  disk  while  giving  attention  to  the 
centre  or  opposite  edge.     This  is  in  accord  with  the  general 
principle  that  peripheral  after-images  are  of  shorter  duration 
than  those  of  the  retinal  centre.     Too  bright  illumination 
should  be  avoided,  for  with  intense  light  the  difference  be- 
tween the  centre  and  periphery  is  less,  or  even  quite  reversed. 

Bellarminow.  On  rotating  disks  and  their  phenomena  in  general, 
see  Helmholtz,  A,  480-501,  Fr.  445-471  (337-357). 

BINOCULAR  PHENOMENA  OF  LIGHT  AND  COLOR.* 

162.  In  general  the  two  eyes  co-operate  to  bring  about 
a  single  visual  result,  but  the  union  of  the  impressions  upon 
the  two  retinae  is  influenced  by  a  number  of  circumstances. 

1  The  experiments  that  follow  can  all  be  made  with  the  stereoscope,  but  prac- 
tice will  enable  the  experimenter  to  combine  the  diagrams  with  free  eyes,  either 
by  crossing  the  lines  of  sight  (fixating  a  point  nearer  than  the  diagram),  or  by 
making  them  parallel  or  nearly  so  (fixating  a  point  beyond  the  diagram).  This 


SENSATIONS   OF  LIGHT  AND   COLOR.  169 

a.  If  the  stimulus  to  one  eye  is  considerably  stronger 
than  that  to  the  other,  the  sensation  in  the  latter  is  in  most 
cases  totally  suppressed.     Close  one  eye  and  look  at  a  sheet 
of  white  paper  with  the  other,  letting  the  open  eye  move 
about  ffeely.     There  is  no  tendency  for  the  darkened  field 
of  the  closed  eye  to  assert  itself. 

b.  When,  however,  the  effect  of  the  stimulus  in  the  open 
eye  is  somewhat  weakened  by  steady  fixation,  such  a  ten- 
dency is  to  be  observed,  and  the  whole  of  the  field  of  the 
open  eye,  except  a  small  area  about  the  point  fixated,  may 
be  suppressed  from  time  to  time  by  the  dark  field  of  the 
closed  eye.     A  slight  motion  will,  however,  instantly  re- 
store the  first.     See  also  Ex.  127. 

c.  A  field  that  contains  sharply  marked  objects  or  con- 
tours will  generally  triumph  over  one  that  does  not.     Try 
combining  the  letters  below  in  such  a  way  that  the  B's  are 
superposed.     In  this  diagram  the  white  field  of  either  eye, 
which  corresponds  to  A  or  C  in  the  other  eye,  will  generally 
not  triumph  over  the  letter. 

AB      BC 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.);  Hering,  P,  380-385;  Aubert, 
A,  550-553;  Wundt,  A,  3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

163.  Fechner's  Paradoxical  Experiment.  Hold  close  be- 
fore one  eye  a  dark  glass,  such  as  is  used  in  protecting  the 
eyes,  or  a  piece  of  ordinary  glass  moderately  smoked  over, 
or  even  a  black  card  with  a  good-sized  pin-hole  in  it,  allow- 
ing the  other  eye  to  remain  free.  It  is  easy  to  see  that  the 

skill  the  experimenter  should  try  to  acquire.  In  these  experiments  it  is  impor- 
tant that  the  eyes  should  be  of  approximately  equal  power;  and  if  the  poorer  eye 
cannot  be  helped  with  lenses,  the  vision  of  the  other  must  be  somewhat  reduced 
by  the  interposition  of  a  sufficient  number  of  plates  of  ordinary  glass. 


170        LABORATORY  COURSE  IN  PSYCHOLOGY. 

binocular  field  is  darkened  by  the  interposition  of  the  dark 
glass.  If,  however,  the  eye  behind  the  glass  is  closed,  or 
the  light  wholly  cut  off  from  it  by  holding  a  black  card  in 
front  of  the  glass,  the  field  appears  decidedly  brighter ;  that 
is  to  say,  cutting  off  a  portion  of  the  stimulus  received  by 
the  total  visual  apparatus,  has  caused  an  increased  intensity 
of  sensation.  The  experiment  fails  for  very  dark  and  very 
light  glasses.  Several  explanations  have  been  given,  but 
that  of  Aubert  (according  to  which  the  sensations  of  the 
two  retinae  blend  in  a  sort  of  average  result  when  the  dif- 
ference is  not  too  great,  but  one  wholly  suppresses  the  other 
when  the  difference  is  very  great)  seems  to  be  the  most 
satisfactory. 

Fechner,  B,  416  ff. ;  Helmholtz,  A,  Fr.  993-994  (790-791) ;  Bering, 
Q,  311  f.;  Aubert,  A,  499-503. 

164.  Rivalry.  When  the  two  retinae  are  stimulated  at 
the  same  time  separately  with  strong  light  of  different 
colors,  or  are  confronted  with  otherwise  incongruous  fields, 
i.e.,  fields  that  cannot  be  given  a  unitary  interpretation, 
there  results  a  peculiar  instability  and  irregular  alternation 
of  the  colors  over  part  or  the  whole  of  the  combined  fields 
of  vision.  This  apparent  struggle  of  the  fields  is  known 
as  Retinal  Rivalry.  Hold  close  before  one  eye  a  piece  of 
blue  glass,  before  the  other  a  piece  of  red  glass,  and  look 
toward  the  sky  or  a  brightly  lighted  uniform  wall.  The 
struggle  of  colors  will  at  once  begin.  The  same  may  be 
observed  with  a  stereoscope  when  the  usual  paired  photo- 
graphs are  replaced  by  colored  fields,  or  even  with  no  ap- 
paratus at  all,  when  both  eyes  are  closed  and  turned  toward 
a  bright  sky  and  one  of  them  is  covered  with  the  hand. 
Lcng  looking  generally  tends  to  quiet  the  rivalry.  Rivalry 
has  been  explained  as  due  to  fluctuations  of  attention,  and 
some  observers  find  that  it  can  be  more  or  less  controlled 
by  attention  (Helmholtz).  Fechner  discusses  the  attention 


OF  THR 

UNIVERSITY 


SENSATIONS  OF  LIGHT 


theory,  and  finds  it  insufficient.  Von  Bezold  thinks  rivalry 
associated  with  changes  in  accommodation  which  follow 
attention.  Hering  and  others  regard  the  changes  as  of 
more  purely  jphysiological  .origin.  See  also  Ex.  165  b. 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.),  974  ff.  (775  ff.);  Hering,  P, 
380-385,  Q,  308  ff.;  Aubert,  A,  550  ff.;  Wundt,  A,  3te  Aufl.,  II. , 
185  ff.,  4te  Aufl.,  II.,  211  ff.;  Chauveau,  C. 

165.  Prevalence  and  Rivalry  of  Contours.  By  contours 
is  here  meant  lines  of  separation  where  fields  of  one  color 
border  upon  fields  of  another  color. 

a.  Combine  stereoscopically  the  two  bars  below,  and  notice 
that  it  is  the  contours  that  suppress  the  solid  parts  of  both 
the  black  and  white.  This  figure  gives  excellent  results 
also  when  colors  are  substituted  for  the  black  and  white. 


Notice  a  similar  triumph  of  the  contours  of  the  cross  in 
the  left-hand  figure  below,  or,  better  still,  in  an  enlargement 
of  it. 


b.  Notice  the   rivalry  of  the  contours  in  all  of  these 
figures. 


172        LABORATORY   COURSE  IN  PSYCHOLOGY. 

c.  The  last  two  pairs  of  diagrams  are  suitable  for  the 
study  of  the  part  played  by  attention  in  rivalry.     While 
it  is  doubtful  whether  mere  attention  to  one  field  or  the 
other  can  cause  it  to  predominate,  it  yet  seems  possible  by 
indirect  application  of  attention  to  cause  it  to  do  so.     If 
attention  is  given  to  an  examination  of  the  lines  and  small 
squares  in  the  left-hand  figure,  or  if  one  of  the  sets  of 
lines  in  the  right-hand  figure  is  counted,  both  will  appear 
to  be  somewhat  assisted  in  their  struggle  with  the  cross  or 
the  other  set  of  lines. 

d.  A  printed  page  has  a  decided  advantage.     Try  a  dia- 
gram in  which  a  printed  page  is  put  in  rivalry  with  a  field 
of  heavy  cross  lines.     The  lines  will  be  found-  .to  yield  -to* 
the  print,  at  least  at  the  point  at  which  the  reader  is  look- 
ing at  the  instant.     Two  printed  pages,  however,  become 
hopelessly  mixed ;  and  it  is  hard  to  say  how  much  of  the 
advantage,  when  a  single  one  is  used,  is  due  to  its  superior 
power  as  a  holcler^  of  ^attention,  and  how  much  to  its  excel- 
lence as  a  set  of   contours.     A  portion  of  the  power  of 
contours  is  probably  to  be  explained  by  the  mutual  intensi- 
fication of  both  the  black  and  the  white  by  contrast  j  but  a 
part  is  perhaps  due  to  a  strong  tendency,  observable  in 
other  cases  also,  for  the  eyes  (and  attention)  to  follow  lines, 
and  especially  outlines. 

Helmholtz,  A,  Fr.  964  ff.  (767  ff.);  Bering,  P,  380-385,  Q,  314: 
Wundt,  A,  3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

166.  Luster.  Sheen.  When  one  of  the  rival  fields  is 
white  and  the  other  colored  (especially  when  one  is  white 
and  the  other  is  black),  there  results,  besides  the  rivalry, 
a  curious  illusion  of  shine  or  polish,  known  as  binocular 
lustre. 

a.  Examine  in  the  stereoscope  a  diagram  made  like  the 
accompanying  cut,  and  notice  the  graphite-like  shine  of  the 


SENSATIONS   OF  LIGHT  AND   COLOR.  173 

pyramid.     The  explanation  seems  to  be  that  polished  sur- 
faces, which  at  some  angles  reflect  light  enough  to  look 


white,  and  at  others  appear  in  their  true  color,  have  often 
in  previous  experience  given  rise  to  such  differences  of  sen- 
sation in  the  two  eyes,  and  from  this  difference  it  is  inferred 
that  the  object  seen  in  the  diagram  is  shiny. 

b.  A  species  of  monocular  lustre  (or  transparence)  is  to 
be  observed  when  black  or  white  or  colors  are  combined  by 
means  of  the  reflection  color-mixer,  especially  when  the 
inclination  of  the  plate  is  so  changed  that  one  color  ap- 
pears to  be  reflected  in  the  surface  of  the  other,  or  to  be 
seen  through  and  behind  it.  The  experiment  works  well 
when  real  objects  are  reflected  in  the  surface  of  the  glass, 
the  reflecting  power  of  the  latter  appearing  to  be  trans- 
ferred to  the  horizontal  surface  on  the  opposite  side. 

Helmholtz,  A,  Fr.  983  ff.  (782  ff.);  Bering,  P,  576-577;  Aubert, 
A,  550  ff.;  Wundt,  A,  3te  Aufl.,  II.,  177  ff.,  183  ff.,  4te  Aufl.,  II., 
204  ff.,  209  ff. 

167.  Binocular  Color  Mixing.  The  result  of  simultane- 
ous presentation  of  different  colors  to  the  two  eyes  is  not 
always  rivalry  or  lustre.  If  the  colors  are  not  too  bright 
and  saturated,  and  the  fields  are  without  fleck  or  spot  to 


174        LABORATORY  COURSE  IN  PSYCHOLOGY. 

give  one  the  predominance,  a  veritable,  though  somewhat 
unsteady,  mixture  of  the  colors  may  result. 

a.  Place  a  red  and  a  blue  glass  of  equal  transparency 
in  the  binocular  color-mixer,  and  adjust  the  side  screens  till 
the  proper  amount  of  white  light  is  mixed  in  with  that 
transmitted  from  below.     The  mixture  will  then  be  seen  on 
the  white  field  below.     Try  also  with  other  combinations  of 
glasses.     Mixtures  obtained  in  this  way  are  not  always  the 
same   in   appearance   as  the   monocular   mixtures   studied 
above,  and  some  observers  have  great  difficulty  in  getting 
them  satisfactorily.     Long^juid-steady -gazing,  which  inter- 
feres with  rivalry,  favors  binocular  color  mixing. 

b.  The  same  effect  may  be  conveniently  obtained  with  a 
stereoscope,   from   which   the   middle   partition   has    been 
removed.     Try  with  equal  areas  of  dull  colors  of  little  satu- 
ration.    Hering  recommends  two  squares  of  red  and  two  of 
blue,  set  at  equal  distances  in  a  horizontal  line,  the  two  reds 
on  one  side,  the  two  blues  on  the  other.     When  the  middle 
pair  are  combined  stereoscopically,  they  show  a  mixed  color, 
while  the  unmixed  colors  can  be  seen  for  comparison  beside 
them.     He  also  suggests  the  use  of  lenses  to  prevent  sharp 
focusing  of  the  eyes  upon  the  contours,  which  interferes 
with  the  mixture.     Complementary  colors  are  said  to  be 
more  difficult  to  fuse  than  those  standing  nearer  in  the 
color  scale.     The  same  is  true  of  colors  differing  greatly  in 
brightness  ;  see  Ex.  163. 

Helmholtz,  A,  Fr.  976  ff.  (776  ff.);  Hering,  P,  591-600;  von 
Bezold,  C;  Chauveau,  A;  Aubert,  A,  550  ff.;  Wundt,  A,  3te  Aufl., 
II.,  183  ff.,  4te  Aufl.,  209  ff. 

168.  Binocular  Contrast.  The  Side- Window  Experiment. 
Stand  so  that  the  light  from  the  window  falls  sidewise  into 
one  eye,  but  not  at  all  into  the  other.  Place  in  a  convenient 
position  for  observation  a  strip  of  white  paper  on  a  black 
surface.  The  paper  when  looked  at  with  both  eyes  appears 


SENSATIONS   OF  LIGHT  AND   COLOR.  175 

perfectly  colorless.  On  looking  now  at  a  point  nearer  than 
the  strip  of  paper  (e.g.,  at  the  finger  held  up  before  the  face), 
double  images  of  the  strip  will  be  seen.  The  two  images  will 
be  different  in  brightness  and  slightly  tinged  with  comple- 
mentary colors.  The  image  belonging  to  the  eye  next  the 
window  (which  may  be  recognized  by  its  disappearance 
when  that  eye  is  closed)  will  appear  tinged  with  a  faint 
blue  or  blue-green  color,  the  other  with  a  very  faint  red  or 
yellow.  The  light  that  enters  the  eye  through  the  sclerotic 
is  tinged  reddish  yellow,  and  makes  the  eye  less  responsive 
to  that  color ;  the  white  of  the  paper  strip  therefore  appears 
bluish.  It  appears  darker  partly  for  a  similar  reason,  and 
perhaps  also,  as  Fechner  suggests,  because  it  lies  in  a  field 
which,  for  the  eye  in  question,  is  generally  bright.  The 
reddish  color  of  the  other  eye's  image  of  the  strip  is  ex- 
plained as  due  to  contrast  with  the  first ,  but  whether  this 
contrast  color  is  a  psychical  matter,  or  whether  it  is  to  be 
explained  by  the  action  of  the  stimulus  in  the  first  eye 
upon  the  second,  as  there  seems  some  reason  to  think,  is  as 
yet  uncertain.  Its  greater  brightness  is  probably  due  to 
the  fresher  condition  of  the  eye  to  which  it  belongs,  and  to 
contrast  with  its  less  brilliant  field.  The  same  thing  is 
often  to  be  noticed  when  reading  with  the  lamp  at  one  side, 
or  even  when  one  eye  has  been  closed  for  a  short  time 
while  the  other  has  been  open.  The  double  images  are 
in  no  wise  essential ;  simple  alternate  winking  will  show 
decided  differences  in  the  condition  of  the  two  eyes. 

Fechner,  B,  511  ff.;  Brucke,  420  ff.;  Bering,  P,  600-601;  Helm- 
holtz,  A,  Fr.  987  ff.  (785  ff.);  Chauveau,  B\  Titchener;  Wundt,  A, 
3te  Aufl.,  II.,  183  ff.,  4te  Aufl.,  II.,  209  ff. 

169.  Binocular  After-images.  Lay  a  bit  of  orange-colored 
paper  on  a  dark  ground,  and  provide  two  white  cards.  Hold 
one  of  the  cards  close  to  the  left  eye,  but  a  little  to  one 
side,  so  as  not  to  hide  the  bit  of  paper.  Hold  the  other 


176        LABORATORY  COURSE  IN  PSYCHOLOGY. 

eight  or  ten  inches  from  the  right  eye  in  such  a  way  as  to 
hide  the  paper.  Look  at  the  paper  for  a  few  seconds  with 
the  left  eye,  then  bring  the  card  before  it.  A  faint,  washy, 
orange-colored  positive  after-image  will  appear  on  the  card 
before  the  right  eye.  The  image  is  by  no  means  easy  to 
observe.  It  is  supposed  to  belong  to  the  right  eye's  half 
of  the  visual  apparatus,  possibly  to  the  central,  i.e.,  cerebral, 
part. 

Ebbinghaus,  C ;  Chauveau,  B  ;  Titchener. 


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SENSATIONS   OF  LIGHT  AND   COLOR.  177 

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ibid.,  394-398. 

C.  Sur  la  theorie  de  1'antagonisme  des  champs  visuels,  ibid.,  439- 
442. 

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178        LABORATORY   COURSE  IN 

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Akademie  zu  Berlin,  1.,  Dec.,  1887. 

C.  Ueber  Nachbilder  im  binocularen  Sehen  und  die  binocularen 
Farbenerscheinungen    uberhaupt,   Pfluger's    Archiv,  XLVI., 
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Pfliiger's  Archiv,  III.,  1870,  214-240. 
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Centralbl,  VI.)  ;  XL.,  1887,  323-330. 

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B.   Ueber  einige  Verhaltnisse  des  Binocularen  Sehens,  Abhl.  d. 
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Pfluger's  Archiv,  XL VII.,  1890,  274-285. 
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Archiv,  XVII.,  1878,  152-153. 

B.   Studien   iiber  Licht-  und   Farbenempfindung,   ibid.,  XLIII., 
1888,  441-501. 

FRANKLIN,  Christine  Ladd:  A.  Eine  neue  Theorie  der  Lichtem- 
pfindung,  Zeitschriftfur  Psychologic,  IV.,  1892,  211-221.  Ab- 
stracts of  this  paper  may  be  found  in  Proc.  Congr.  Exper.  Psy., 
London,  1892;  Johns  Hopkins  University  Circulars,  XII.,  No. 
106,  June,  1893,  108-110;  Science,  XXII.,  July  14,  1893,  18-19. 
B.  On  Theories  of  Light  Sensation,  Mind,  Ser.  2,  II.,  1893,  473- 
489. 

HELMHOLTZ:  A.     Work  cited  with  same  letter  in  bibliography  of 
Chap.  V. 

B.  Popular  Scientific  Lectures,  First  Series,  New  York,  1885. 

C.  Versuch  einer  erweiterten  Anwendung  des  Fechnerschen  Ge- 
setzes  im  Farbensystem,  Zeitschrift  fur  Psychologic,  II.,  1892, 
1-30. 


SENSATIONS   OF  LIGHT  AND   COLOR.  179 

_D.  Yersuch  das  psychophysische  Gesetz  auf  die  Farbenunter- 
schiede  trichromatischer  Augen  anzuwenden,  ibid.,  III.,  1891, 
1-20. 

E.     Kiirzeste  Linien  im  Farbensystem,  ibid.,  108-122.    An  extract 

from  Sitz.-ber.  der  Akademie  zu  Berlin,  17,  December,  1891. 
HERiNG:1  A.     Zur  Lehre  vom  Lichtsinne,  Wien,  1878.    Keprint  of 
six  communications  to  the  Vienna  Academy,  1872-74.    For  an 
extended  abstract  of  this  work,  made  by  Dr.  William  Pole,  see 
Nature,  XX.,  1879,  611-613,  637-639;  XXI.,  1879-80,  14-17. 

B.  Zur  Erklarung  der  Farbenblindheit  aus  der  Theorie  der  Gegen- 
farben,  Prag,    1880.     Keprint  from  Lotos,  Neue  Folge,  I.,  1880. 

C.  Ueber  individuelle  Yerschiedenheiten  des  Farbensinns,  Lotos, 
Neue  Folge,  VI.,  1885. 

D.  Beleuchtung  eines  Angriffes  auf  die  Theorie  der  Gegenf arben, 
Pfluger's  Archiv,  XLL,  1887,  29-46. 

E.  Ueber  die  Theorie  des  simultanen  Contrastes  von  Helmholtz, 
ibid.,  XL.,  1886-87,  172-191    (Die   farbigen   Schatten);    XLL, 
1887,  1-29   (Der  Contrastversuch  von  H.  Meyer  und  die  Yer- 
suche  am  Farbenkreisel) ;  358-367  (Der  Spiegelcontrastversuch) ; 
XLIIL,  1888,  1-21  (Die  subjective  „  Trennung  des  Lichtes  in 
zwei  complementare  Portionen  "). 

F.  Ueber  die  von  v.  Kries  wider  die  Theorie  der  Gegenfarben 
erhobenen  Einwande,  ibid.,  XLIL,  1888,  488-506;  XLIIL,  1888, 
264-288,  329-346. 

G.  Ueber  die  Hypothesen  zur  Erklarung  der  peripheren  Farben- 
blindheit, v.  Graefe's  Archiv,  XXXV.,  1889,  iv.,  63-83;  XXXVL, 
1890,  L,  264. 

H.  Zur  Diagnostik  der  Farbenblindheit,  ibid.,  XXX VI.,  1890,  i., 
217-233. 

J.  Die  Untersuchung  einseitiger  Storungen  des  Farbensinnes  mit- 
tels  binocularer  Farbengleichungen,  ibid.,  XXXVI. ,  1890,  iii., 
1-23. 

J.  Beitrag  zur  Lehre  vom  Simultankontrast,  Zeitschrift  fur  Psy- 
chologic, L,  1890,  18-28. 

1  Herlng's  work  upon  color  has  not  yet  been  gathered  into  one  consecutive 
whole.  It  has  seemed  well,  therefore,  to  insert  here,  in  addition  to  the  titles  of 
papers  bearing  directly  on  the  experiments  of  Chap.  VI.,  such  other  titles  on 
light  and  color  as  came  to  hand. 


180       LABORATORY  COURSE  IN  PSYCHOLOGY. 

IT..  Eine    Methode    zur    Beobachtung    des    Simultancontrastes, 

Pfluger's  Archiv,  XLYII.,  1890,  236-242. 
L.   Priifung  der  sogenannten  Farbendreiecke  mit  Hiilfe  des  Farben- 

sinns  excentrischer  Netzhautstellen,  ibid.,  XLYII.,  1890,  417- 

438. 
M.   Ueber   Newton's    Gesetz    der  Farbenmischung,  Prag,    1887. 

Reprint  from  Lotos,  VII.,  1887. 
N.   Untersuchung  eines  total   Farbenblinden,  Pfluger's  Archiv, 

XLIX.,  1891,  563-608. 
O.   Eine  Vorrichtung  zur  Farbenmischung,  zur  Diagnose  der  Far- 

benblindheit  und  zur  Untersuchung  der  Contrasterscheinungen, 

Pfluger's  Archiv,  XLII.,  1888,  119-144. 
P.   Work  cited  with  reference  letter  A  in  the  bibliography  of 

Chap.  V. 
Q.   Work  cited  with  reference  letter  B  in   the  bibliography  of 

Chap.  V. 
R.   Ueber    Holmgren's    vermeintlichen    Nachweis   der    Elemen- 

tarempfindungen    des    Gesichtssinns,    Pjluger^s   Archiv,    XL., 

1887,  1-20. 
S.   Kritik  einer  Abhandlung  von  Bonders,  Lotos,  Neue  Folge,  II., 

Prag,  1882. 
T.   Ueber    Sigmund    Exner's   neue   Urtheilstauschung  auf   dem 

Gebiete  des  Gesichtsinnes,  Pfliiger's  Archiv,   XXXIX.,  1886, 

159-170. 
U.   Ueber  den  Begriff  „  Urtheilstauschung "   in  der  physiologi- 

schen  Optik  und  iiber  die  Wahrnehmung  simultaner  und  suc- 

cessiver  Helligkeitsunterschiede,  ibid.,  XLL,  1887,  91-106. 

HESS:  A.  Ueber  den  Farbensinn  bei  indirectem  Sehen,  v.  Graefe's 
Archiv,  XXXV.,  1889,  iv.,  1-62. 

B.  Untersuchung  eines  Falles  von  halbseitiger  Farbensinnsstorung 
am  linken  Auge,  ibid.,  XXXVI.,  1890,  iii.,  24-36. 

C.  Ueber  die  Tonanderungen  der  Spektralfarben  durch  Ermiidung 
der  Netzhaut  mit  homogenem  Lichte,  ibid.,  XXXVI.,  1890,  i., 
1-32. 

HILLEBRAND  :  Work  cited  in  bibliography  of  Chap.  V. 

HOLMGREN  :  Color-blindness  in  its  Relation  to  Accidents  by  Rail  and 
Sea.  Translation  by  M.  L.  Duncan,  Smithsonian  Report,  1877, 
131-195. 


SENSATIONS  OF  LIGHT  AND  COLOR.  181 

JASTBOW:  A  Novel  Optical  Illusion,  American  Journal  of  Psy- 
chology, IV.,  1891-92,  201-208. 

JEFFRIES:  A.  Color-blindness,  its  Dangers  and  its  Detection,  Bos- 
ton, 1879.  This  work  contains  a  seventeen-page  bibliography 
on  .color-blindness  and  kindred  topics. 

B.   Color-blindness,  Article   in  the  Keference  Handbook  of  the 
Medical  Sciences,  New  York,  1886,  II.,  241. 

KIRSCHMANN:  A.  Beitrage  zur  Kenntniss  der  Farbenblindheit, 
WundVs  Philos.  Studien,  VIII.,  1892-93,  173-230,  407-430. 

B.  Ueber  die  Helligkeitsempfindung  im  indirecten  Sehen,  ibid., 
V.,  1889,  447-497. 

C.  Die  Farbenempfindung  im  indirecten  Sehen,  Erste  Mittheilung, 
ibid.,  VIII.,  1892-93,  592-614. 

D.  Ueber  die  quantitativen  Verhaltnisse  des  simultanen  Hellig- 
keits-  und  Farben-Contrastes,  ibid.,  VI.,  1890,  417-491. 

E.  Some  Effects  of  Contrast,  American  Journal  of  Psychology, 
IV.,  1892,  542-557. 

KONIG:  A.  Ueber  den  Helligkeitswert  der  Spektralfarben  bei 
verschiedener  absoluter  Intensitat,  Beitrage  zur  Psychologic 
und  Physiologic  der  Sinnesorgane  (Helmholtz  Festgruss),  Ham- 
burg und  Leipzig,  1891,  311-388. 

B.  The  Modern   Development   of    Thomas  Young's  Theory  of 
Colour  Vision,   Report   of  British  Association,  Birmingham 
Meeting,  1886,  431-439. 

C.  Zur  Kenntniss  dichromatischer  Farbensysteme,  Wiedemann's 
Annalen,  XXII.,  1884,  567-578. 

KONIG  UND  DIETERICI:  A.  Die  Grundempfindungen  in  normalen 
und  anomalen  Farbensystemen  und  ihre  Intensitatsverteilung 
im  Spektrum,  Zeitschrift  fur  Psychologie,  IV.,  1892,  241-347. 
B.  Ueber  die  Empfindlichkeit  des  normalen  Auges  fur  Wellen- 
langenunterschiede  des  Lichtes,  Wiedemann's  Annalen,  XXII., 
1884,  579-589. 

VON  KRIES  :  Die  Gesichtsempfindungen  und  ihre  Analyse,  Du  Bois- 
Reymond's  Archiv,  1882,   Supplement-Band,  1-178.     A  care- 
ful summary  and  discussion  of  the  whole  subject. 
LEHMANN  :  Ueber  die  Anwendung  der  Methode  der  mittleren  Abstu- 
fungen  auf  den  Lichtsinn;  die  quantitative  Bestimmung  des 
Lichtcontrastes,  WundVs  Philos.  Studien,  III.,  1886,  516-528. 


182        LABORATORY  COURSE  IN  PSYCHOLOGY 

MAXWELL  :  A.  On  the  Theory  of  Compound  Colours,  and  the  Relation 

of  the  Colours  of  the  Spectrum,  Phil.  Trans.,  CL.,  1860,  57-84. 
B.   On  Colour  Vision,  Proc.  Royal  Institution  of  Great  Britain,  VI. 

These  two  papers  are  also  to  be  found  in  Maxwell's  Scientific 

Papers,  Cambridge,  1890,  I.,  410-440,  II.,  267-280. 
MAYER:  Studies  of  the  phenomena  of  Simultaneous  Contrast-Color; 

and  on  a  Photometer  for  measuring  the  intensities  of  Lights  of 

different  colors,  American  Journal  of  Science,  Ser.  3,  XLVL, 

1893,  1-22;  also  Phil.  Mag.,  Ser.  5,  XXXVI.,  1893,  153-175. 
MEYER:  Ueber  Contrast-  oder  Complementarfarben,  Poggendorff's 

Annalen,  XCV.,  1855,  170-171;   also  Phil.  Mag.,  Ser.  4,  IX., 

Jan.-June,  1855,  547. 
NICHOLS:  A.  On  the  Sensitiveness  of  the  Eye  to  Colors  of  a  Low 

Degree  of  Saturation,    American  Journal  of  Science,  Ser.   3, 

XXX.,  1885,  37-41. 
B.   Duration  of  Color  Impressions   upon  the   Retina,  American 

Journal  of  Science,  Ser.  3,  XXVIII. ,  1884,  243-252. 
PACE:    Zur  Frage  der  Schwankungen   der  Aufmerksamkeit  nach 

Versuchen   mit   der  Masson'schen    Scheibe,    Wundfs   PMlos. 

Studien,  VIII.,  1892-93,  388-402. 

PEIRCE,  B.  O.,  JR.  :  On  the  Sensitiveness  of  the  Eye  to  Slight  Differ- 
ences of  Color,  American  Journal  of  Science,  Ser.  3,  XXVI. , 
1883,  299-302. 
PEIRCE,  C.  S. :  Note  on  the  Sensation  of  Color,  American  Journal 

of  Science,  Ser.  3,  XIIL,  1877,  247-251. 

PLATEAU:  Betrachtungen  iiber  ein  von  Hrn.  Talbot  vorgeschla- 
genes  photometrisches  Pr'mcip,Poggendorff's  Annalen,  XXXV., 
1835,  457-468. 

POLE:  Further  Data  on  Colour-Blindness,  P/i^.Jfa<?.,  Ser.  5,XXXIV., 
1892,  100-114,  439-443,  XXXV.,  1893,  52-62,  XXXVI.,  1893, 
188-195. 

PREYER:  Work  cited  in  bibliography  of  Chap.  I. 
RAYLEIGH:  A.   Experiments  on  Colour,  Nature,  XXV.,   1881-82, 

64-66. 

B.  Rayleigh  and  others:  Report  of  the  [Royal  Society's]  Commit- 
tee on  Colour- Vision,  Proc.  Roy.  Soc.,  LI.,  No.  311,  July  19, 
1892,  281-396. 


SENSATIONS   OF  LIGHT   AND  COLOR.  183 

ROOD:  A.   Students'  Textbook  of  Color,  New  York,  1881. 

B.    On  a  new  Theory  of  Light,  proposed  by  John  Smith,  M.A., 

American  Journal  of  Science,  Ser.  2,  XXX.,  1860,  182-186. 
SCHUSTER:  Experiments  with  Lord  Rayleigh's  Colour  Box,  Proc. 

RO&  Soc.,  XL VIII.,  1890,  140-149. 
TALBOT:  Experiments  on  Light,  Phil.  Mag.,  Ser.  3,  V.,  July-Dec., 

1834,  321-334,  especially,  327-334. 
TITCHENER:   Ueber     binoculare    Wirkungen     monocularer    Reize, 

WundV s  Philos.  Studien,  VIII.,  1892-93,  231-310.     Cites  liters 

ture. 
WUNDT:  A.   Work  cited  with    same  letter  in  the  bibliography  of 

Chap.  V. 
B.  Die  Empfindung  des  Lichts  und  der  Farben,  Wundfs  Philos. 

Studien,  IV.,  1888,  311-389. 

For  further  bibliographical  references,  see  the  works  of  Helmholtz 
and  Aubert  and  the  following  by  Plateau :  Bibliographic  analytique 
des  principaux  phenomenes  subjectifs  de  la  vision,  depuis  les  temps 
anciens  jusqu'a  la  fin  du  XVIII.  siecle;  suivie  d'une  bibliographic 
simple  pour  la  partie  ecoulee  du  siecle  actuel.  Mem.  cour.  de  1'Acad. 
R.  de  Belgique;  Bruxelles,  1876-77. 


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Wright's  Nature  Reader,  NO.  II.  Describes  ants,  flies,  earth-worms,  beetles,  bar- 
nacles and  star-fish.  Boards.  40  cts. 

Wright's  Nature  Reader,  NO.  III.  Has  lessons  in  plant-life,  grasshoppers,  butter 
flies,  and  birds.  Boards.  60  cts. 

Wright's  Nature  Reader,  NO.  IV.  Has  lessons  in  geology,  astronomy,  world-life, 
etc.  Boards.  70  cts. 

For  advanced  supplementary  reading  see  our  list  of  books  in  English  Literature. 


D.    C.    HEATH    &    CO.,    PUBLISHERS, 

BOSTON.        NEW  YORK.       CHICAGO. 


NUMBER. 


AtWOOd's  Complete  Graded  Arithmetic.  Present  a  carefully  graded  course  in 
arithmetic,  to  begin  with  the  fourth  year  and  continue  through  the  eiehth  vear  Part  T 
200  pages.  Cloth.  40  cts.  Part  II.  382  pages.  Half  leather.  75  cts. 

Walsh's   Mathematics  for  Common  Schools.      Special  features  of  this  work  are 


75  cts, 

Sutton  and  Kimbrough's  Pupils'  Series  of  Arithmetics. 

PRIMARY  Book.     Embraces  the  four  fundamental  operations  in  all  their  simple  relations. 

80  pages.     Cloth.     25  cts. 
INTERMEDIATE  BOOK.    Embraces  practical  work  through  percentage  and  simple  interest 

145  pages.     Cloth.     30  cts. 

LOWER  BOOK.     Primary  and  Intermediate  Books  bound  together.     Cloth.     45  cts 
HIGHER   BOOK.     A  compact  volume  for  efficient  work  which  makes  clear  all  necessary 

theory.    275  pages.    Half  leather.    75  cts. 

Safford's  Mathematical  Teaching.      Presents  the  best  methods  of  teaching,  from 
primary  arithmetic  to  the  calculus.     Paper.     25  cts. 

Badlam'S   Aids  tO    Number.      For  Teachers.     First  Series.     Consists  of  25  cards  for 
sight-work  with  objects  from  one  to  ten.    40  cts. 

Badlam'S   Aids  tO   Number.      For  Pupils.     First  Series.     Supplements   the  above 
with  material  for  slate  work.     Leatherette.     30  cts. 

Badlam'S  Aids   tO   Number.      For  Teachers.     Second  Series.     Teachers'  sight-work 
with  objects  above  ten.    40  cts. 

Badlam'S  Aids   tO   Number.      For  Pupils.     Second  Series.     Supplements  above  with 
material  for  slate  work  from  10  to  20.     Leatherette.     30  cts. 

Badlam'S  Number  Chart.      n  x  14  inches.    Designed  to  aid  in  teaching  the  four 
fundamental  rules  in  lowest  primary  grades.     5  cts.  each ;  per  hundred  $4.00. 

Sloane'S    Practical    Lessons    in    Fractions.      For  elementary  grades.      Boards. 
30  cts.     Set  of  six  fraction  cards  for  children  to  cut.     12  cts. 

White's  TWO  Years  With  Numbers.      Number  Lessons  for  second  and  third  year 
pupils.     40  cts. 

White's  Junior  Arithmetic.      For  fourth  and  fifth  year  pupils.     Cloth.     50  cts. 

White's  Senior  Arithmetic,    inpress. 

For  advanced  work  see  our  list  of  books  in  Mathematics. 


D.   C.  HEATH   &   CO.,  PUBLISHERS, 

BOSTON.        NEW  YORK.        CHICAGO. 


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